diff --git a/src/boxes/array/mod.rs b/src/boxes/array/mod.rs
index 4cef49f4..33853490 100644
--- a/src/boxes/array/mod.rs
+++ b/src/boxes/array/mod.rs
@@ -1,15 +1,14 @@
 //! ArrayBox 📦 - 配列・リスト操作
 // Nyashの箱システムによる配列・リスト操作を提供します。
-// Arc<Mutex>パターンで内部可変性を実現
+// RwLockパターンで内部可変性を実現（Phase 9.75-B Arc<Mutex>削除）
 
 use crate::box_trait::{NyashBox, StringBox, BoolBox, IntegerBox, BoxCore, BoxBase};
 use std::any::Any;
-use std::sync::{Arc, Mutex};
+use std::sync::RwLock;
 use std::fmt::Display;
 
-#[derive(Debug, Clone)]
 pub struct ArrayBox {
-    pub items: Arc<Mutex<Vec<Box<dyn NyashBox>>>>,
+    pub items: RwLock<Vec<Box<dyn NyashBox>>>,
     base: BoxBase,
 }
 
@@ -17,7 +16,7 @@ impl ArrayBox {
     /// 新しいArrayBoxを作成
     pub fn new() -> Self {
         ArrayBox { 
-            items: Arc::new(Mutex::new(Vec::new())),
+            items: RwLock::new(Vec::new()),
             base: BoxBase::new(),
         }
     }
@@ -25,20 +24,20 @@ impl ArrayBox {
     /// 要素を持つArrayBoxを作成
     pub fn new_with_elements(elements: Vec<Box<dyn NyashBox>>) -> Self {
         ArrayBox { 
-            items: Arc::new(Mutex::new(elements)),
+            items: RwLock::new(elements),
             base: BoxBase::new(),
         }
     }
     
     /// 要素を追加
     pub fn push(&self, item: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
-        self.items.lock().unwrap().push(item);
+        self.items.write().unwrap().push(item);
         Box::new(StringBox::new("ok"))
     }
     
     /// 最後の要素を取り出す
     pub fn pop(&self) -> Box<dyn NyashBox> {
-        match self.items.lock().unwrap().pop() {
+        match self.items.write().unwrap().pop() {
             Some(item) => item,
             None => Box::new(crate::boxes::null_box::NullBox::new()),
         }
@@ -46,19 +45,19 @@ impl ArrayBox {
     
     /// 要素数を取得
     pub fn length(&self) -> Box<dyn NyashBox> {
-        Box::new(IntegerBox::new(self.items.lock().unwrap().len() as i64))
+        Box::new(IntegerBox::new(self.items.read().unwrap().len() as i64))
     }
 
     /// Rust向けヘルパー: 要素数をusizeで取得（テスト用）
     pub fn len(&self) -> usize {
-        self.items.lock().unwrap().len()
+        self.items.read().unwrap().len()
     }
     
     /// インデックスで要素を取得
     pub fn get(&self, index: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(idx_box) = index.as_any().downcast_ref::<IntegerBox>() {
             let idx = idx_box.value as usize;
-            let items = self.items.lock().unwrap();
+            let items = self.items.read().unwrap();
             match items.get(idx) {
                 Some(item) => item.clone_box(),
                 None => Box::new(crate::boxes::null_box::NullBox::new()),
@@ -72,7 +71,7 @@ impl ArrayBox {
     pub fn set(&self, index: Box<dyn NyashBox>, value: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(idx_box) = index.as_any().downcast_ref::<IntegerBox>() {
             let idx = idx_box.value as usize;
-            let mut items = self.items.lock().unwrap();
+            let mut items = self.items.write().unwrap();
             if idx < items.len() {
                 items[idx] = value;
                 Box::new(StringBox::new("ok"))
@@ -88,7 +87,7 @@ impl ArrayBox {
     pub fn remove(&self, index: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(idx_box) = index.as_any().downcast_ref::<IntegerBox>() {
             let idx = idx_box.value as usize;
-            let mut items = self.items.lock().unwrap();
+            let mut items = self.items.write().unwrap();
             if idx < items.len() {
                 items.remove(idx)
             } else {
@@ -101,7 +100,7 @@ impl ArrayBox {
     
     /// 指定された値のインデックスを検索
     pub fn indexOf(&self, value: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
-        let items = self.items.lock().unwrap();
+        let items = self.items.read().unwrap();
         for (i, item) in items.iter().enumerate() {
             if item.equals(value.as_ref()).value {
                 return Box::new(IntegerBox::new(i as i64));
@@ -112,7 +111,7 @@ impl ArrayBox {
     
     /// 指定された値が含まれているか確認
     pub fn contains(&self, value: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
-        let items = self.items.lock().unwrap();
+        let items = self.items.read().unwrap();
         for item in items.iter() {
             if item.equals(value.as_ref()).value {
                 return Box::new(BoolBox::new(true));
@@ -123,14 +122,14 @@ impl ArrayBox {
     
     /// 配列を空にする
     pub fn clear(&self) -> Box<dyn NyashBox> {
-        self.items.lock().unwrap().clear();
+        self.items.write().unwrap().clear();
         Box::new(StringBox::new("ok"))
     }
     
     /// 文字列結合
     pub fn join(&self, delimiter: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(sep_box) = delimiter.as_any().downcast_ref::<StringBox>() {
-            let items = self.items.lock().unwrap();
+            let items = self.items.read().unwrap();
             let parts: Vec<String> = items
                 .iter()
                 .map(|item| item.to_string_box().value)
@@ -143,7 +142,7 @@ impl ArrayBox {
     
     /// 配列をソート（昇順）
     pub fn sort(&self) -> Box<dyn NyashBox> {
-        let mut items = self.items.lock().unwrap();
+        let mut items = self.items.write().unwrap();
         
         // Numeric values first, then string values
         items.sort_by(|a, b| {
@@ -191,14 +190,14 @@ impl ArrayBox {
     
     /// 配列を反転
     pub fn reverse(&self) -> Box<dyn NyashBox> {
-        let mut items = self.items.lock().unwrap();
+        let mut items = self.items.write().unwrap();
         items.reverse();
         Box::new(StringBox::new("ok"))
     }
     
     /// 部分配列を取得
     pub fn slice(&self, start: Box<dyn NyashBox>, end: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
-        let items = self.items.lock().unwrap();
+        let items = self.items.read().unwrap();
         
         // Extract start and end indices
         let start_idx = if let Some(start_int) = start.as_any().downcast_ref::<IntegerBox>() {
@@ -236,6 +235,17 @@ impl ArrayBox {
     }
 }
 
+// Clone implementation for ArrayBox (needed since RwLock doesn't auto-derive Clone)
+impl Clone for ArrayBox {
+    fn clone(&self) -> Self {
+        let items = self.items.read().unwrap();
+        let cloned_items: Vec<Box<dyn NyashBox>> = items.iter()
+            .map(|item| item.clone_box())
+            .collect();
+        ArrayBox::new_with_elements(cloned_items)
+    }
+}
+
 impl BoxCore for ArrayBox {
     fn box_id(&self) -> u64 {
         self.base.id
@@ -246,7 +256,7 @@ impl BoxCore for ArrayBox {
     }
     
     fn fmt_box(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        let items = self.items.lock().unwrap();
+        let items = self.items.read().unwrap();
         let strings: Vec<String> = items.iter()
             .map(|item| item.to_string_box().value)
             .collect();
@@ -274,7 +284,7 @@ impl NyashBox for ArrayBox {
     }
 
     fn to_string_box(&self) -> StringBox {
-        let items = self.items.lock().unwrap();
+        let items = self.items.read().unwrap();
         let strings: Vec<String> = items.iter()
             .map(|item| item.to_string_box().value)
             .collect();
diff --git a/src/boxes/buffer/mod.rs b/src/boxes/buffer/mod.rs
index abc75cef..70788be1 100644
--- a/src/boxes/buffer/mod.rs
+++ b/src/boxes/buffer/mod.rs
@@ -31,31 +31,30 @@
 use crate::box_trait::{NyashBox, StringBox, BoolBox, IntegerBox, BoxCore, BoxBase};
 use crate::boxes::array::ArrayBox;
 use std::any::Any;
-use std::sync::{Arc, Mutex};
+use std::sync::RwLock;
 use std::fmt::{Debug, Display};
 
-#[derive(Debug, Clone)]
 pub struct BufferBox {
-    data: Arc<Mutex<Vec<u8>>>,
+    data: RwLock<Vec<u8>>,
     base: BoxBase,
 }
 
 impl BufferBox {
     pub fn new() -> Self {
         BufferBox { 
-            data: Arc::new(Mutex::new(Vec::new())),
+            data: RwLock::new(Vec::new()),
             base: BoxBase::new(),
         }
     }
 
     /// Rust向けヘルパー: バッファ長をusizeで取得（テスト用）
     pub fn len(&self) -> usize {
-        self.data.lock().unwrap().len()
+        self.data.read().unwrap().len()
     }
     
     pub fn from_vec(data: Vec<u8>) -> Self {
         BufferBox { 
-            data: Arc::new(Mutex::new(data)),
+            data: RwLock::new(data),
             base: BoxBase::new(),
         }
     }
@@ -64,8 +63,8 @@ impl BufferBox {
     pub fn write(&self, data: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         // ArrayBoxから変換 - use crate::boxes::array::ArrayBox directly
         if let Some(array_box) = data.as_any().downcast_ref::<crate::boxes::array::ArrayBox>() {
-            let mut buffer = self.data.lock().unwrap();
-            let items = array_box.items.lock().unwrap();
+            let mut buffer = self.data.write().unwrap();
+            let items = array_box.items.read().unwrap();
             for item in items.iter() {
                 if let Some(int_box) = item.as_any().downcast_ref::<IntegerBox>() {
                     if int_box.value >= 0 && int_box.value <= 255 {
@@ -82,7 +81,7 @@ impl BufferBox {
     
     /// すべてのデータを読み取る
     pub fn readAll(&self) -> Box<dyn NyashBox> {
-        let buffer = self.data.lock().unwrap();
+        let buffer = self.data.read().unwrap();
         let array = ArrayBox::new();
         for &byte in buffer.iter() {
             array.push(Box::new(IntegerBox::new(byte as i64)));
@@ -93,7 +92,7 @@ impl BufferBox {
     /// 指定バイト数読み取る
     pub fn read(&self, count: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(count_int) = count.as_any().downcast_ref::<IntegerBox>() {
-            let mut buffer = self.data.lock().unwrap();
+            let mut buffer = self.data.write().unwrap();
             let count = count_int.value.min(buffer.len() as i64) as usize;
             let array = ArrayBox::new();
             
@@ -110,20 +109,20 @@ impl BufferBox {
     
     /// バッファをクリア
     pub fn clear(&self) -> Box<dyn NyashBox> {
-        self.data.lock().unwrap().clear();
+        self.data.write().unwrap().clear();
         Box::new(StringBox::new("ok"))
     }
     
     /// データサイズを取得
     pub fn length(&self) -> Box<dyn NyashBox> {
-        Box::new(IntegerBox::new(self.data.lock().unwrap().len() as i64))
+        Box::new(IntegerBox::new(self.data.read().unwrap().len() as i64))
     }
     
     /// 他のBufferBoxを追加
     pub fn append(&self, other: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         if let Some(other_buffer) = other.as_any().downcast_ref::<BufferBox>() {
-            let mut self_data = self.data.lock().unwrap();
-            let other_data = other_buffer.data.lock().unwrap();
+            let mut self_data = self.data.write().unwrap();
+            let other_data = other_buffer.data.read().unwrap();
             self_data.extend_from_slice(&other_data);
             Box::new(IntegerBox::new(self_data.len() as i64))
         } else {
@@ -137,7 +136,7 @@ impl BufferBox {
             start.as_any().downcast_ref::<IntegerBox>(),
             end.as_any().downcast_ref::<IntegerBox>()
         ) {
-            let data = self.data.lock().unwrap();
+            let data = self.data.read().unwrap();
             let start = (start_int.value as usize).min(data.len());
             let end = (end_int.value as usize).min(data.len());
             
@@ -153,6 +152,14 @@ impl BufferBox {
     }
 }
 
+// Clone implementation for BufferBox (needed since RwLock doesn't auto-derive Clone)
+impl Clone for BufferBox {
+    fn clone(&self) -> Self {
+        let data = self.data.read().unwrap();
+        BufferBox::from_vec(data.clone())
+    }
+}
+
 impl BoxCore for BufferBox {
     fn box_id(&self) -> u64 {
         self.base.id
@@ -163,7 +170,7 @@ impl BoxCore for BufferBox {
     }
     
     fn fmt_box(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        let data = self.data.lock().unwrap();
+        let data = self.data.read().unwrap();
         write!(f, "BufferBox({} bytes)", data.len())
     }
     
@@ -188,7 +195,7 @@ impl NyashBox for BufferBox {
     }
 
     fn to_string_box(&self) -> StringBox {
-        let data = self.data.lock().unwrap();
+        let data = self.data.read().unwrap();
         StringBox::new(format!("BufferBox({} bytes)", data.len()))
     }
 
@@ -200,9 +207,9 @@ impl NyashBox for BufferBox {
 
     fn equals(&self, other: &dyn NyashBox) -> BoolBox {
         if let Some(other_buffer) = other.as_any().downcast_ref::<BufferBox>() {
-            // Arc<Mutex>の内容を比較
-            let self_data = self.data.lock().unwrap();
-            let other_data = other_buffer.data.lock().unwrap();
+            // RwLock内容を比較
+            let self_data = self.data.read().unwrap();
+            let other_data = other_buffer.data.read().unwrap();
             BoolBox::new(*self_data == *other_data)
         } else {
             BoolBox::new(false)
diff --git a/src/boxes/map_box.rs b/src/boxes/map_box.rs
index c8535d74..d2e31be0 100644
--- a/src/boxes/map_box.rs
+++ b/src/boxes/map_box.rs
@@ -108,19 +108,18 @@ use crate::boxes::ArrayBox;
 use std::fmt::{Debug, Display};
 use std::any::Any;
 use std::collections::HashMap;
-use std::sync::{Arc, Mutex};
+use std::sync::RwLock;
 
 /// キーバリューストアを表すBox
-#[derive(Clone)]
 pub struct MapBox {
-    data: Arc<Mutex<HashMap<String, Box<dyn NyashBox>>>>,
+    data: RwLock<HashMap<String, Box<dyn NyashBox>>>,
     base: BoxBase,
 }
 
 impl MapBox {
     pub fn new() -> Self {
         Self {
-            data: Arc::new(Mutex::new(HashMap::new())),
+            data: RwLock::new(HashMap::new()),
             base: BoxBase::new(),
         }
     }
@@ -128,14 +127,14 @@ impl MapBox {
     /// 値を設定
     pub fn set(&self, key: Box<dyn NyashBox>, value: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         let key_str = key.to_string_box().value;
-        self.data.lock().unwrap().insert(key_str.clone(), value);
+        self.data.write().unwrap().insert(key_str.clone(), value);
         Box::new(StringBox::new(&format!("Set key: {}", key_str)))
     }
     
     /// 値を取得
     pub fn get(&self, key: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         let key_str = key.to_string_box().value;
-        match self.data.lock().unwrap().get(&key_str) {
+        match self.data.read().unwrap().get(&key_str) {
             Some(value) => value.clone_box(),
             None => Box::new(StringBox::new(&format!("Key not found: {}", key_str))),
         }
@@ -144,13 +143,13 @@ impl MapBox {
     /// キーが存在するかチェック
     pub fn has(&self, key: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         let key_str = key.to_string_box().value;
-        Box::new(BoolBox::new(self.data.lock().unwrap().contains_key(&key_str)))
+        Box::new(BoolBox::new(self.data.read().unwrap().contains_key(&key_str)))
     }
     
     /// キーを削除
     pub fn delete(&self, key: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         let key_str = key.to_string_box().value;
-        match self.data.lock().unwrap().remove(&key_str) {
+        match self.data.write().unwrap().remove(&key_str) {
             Some(_) => Box::new(StringBox::new(&format!("Deleted key: {}", key_str))),
             None => Box::new(StringBox::new(&format!("Key not found: {}", key_str))),
         }
@@ -158,7 +157,7 @@ impl MapBox {
     
     /// 全てのキーを取得
     pub fn keys(&self) -> Box<dyn NyashBox> {
-        let keys: Vec<String> = self.data.lock().unwrap().keys().cloned().collect();
+        let keys: Vec<String> = self.data.read().unwrap().keys().cloned().collect();
         let array = ArrayBox::new();
         for key in keys {
             array.push(Box::new(StringBox::new(&key)));
@@ -168,7 +167,7 @@ impl MapBox {
     
     /// 全ての値を取得
     pub fn values(&self) -> Box<dyn NyashBox> {
-        let values: Vec<Box<dyn NyashBox>> = self.data.lock().unwrap()
+        let values: Vec<Box<dyn NyashBox>> = self.data.read().unwrap()
             .values()
             .map(|v| v.clone_box())
             .collect();
@@ -181,25 +180,25 @@ impl MapBox {
     
     /// サイズを取得
     pub fn size(&self) -> Box<dyn NyashBox> {
-        Box::new(IntegerBox::new(self.data.lock().unwrap().len() as i64))
+        Box::new(IntegerBox::new(self.data.read().unwrap().len() as i64))
     }
     
     /// 全てクリア
     pub fn clear(&self) -> Box<dyn NyashBox> {
-        self.data.lock().unwrap().clear();
+        self.data.write().unwrap().clear();
         Box::new(StringBox::new("Map cleared"))
     }
     
     /// 各要素に対して関数を実行
     pub fn forEach(&self, _callback: Box<dyn NyashBox>) -> Box<dyn NyashBox> {
         // 簡易実装：callbackの実行はスキップ
-        let count = self.data.lock().unwrap().len();
+        let count = self.data.read().unwrap().len();
         Box::new(StringBox::new(&format!("Iterated over {} items", count)))
     }
     
     /// JSON文字列に変換
     pub fn toJSON(&self) -> Box<dyn NyashBox> {
-        let data = self.data.lock().unwrap();
+        let data = self.data.read().unwrap();
         let mut json_parts = Vec::new();
         
         for (key, value) in data.iter() {
@@ -218,8 +217,22 @@ impl MapBox {
     }
     
     /// 内部データへのアクセス（JSONBox用）
-    pub fn get_data(&self) -> Arc<Mutex<HashMap<String, Box<dyn NyashBox>>>> {
-        self.data.clone()
+    pub fn get_data(&self) -> &RwLock<HashMap<String, Box<dyn NyashBox>>> {
+        &self.data
+    }
+}
+
+// Clone implementation for MapBox (needed since RwLock doesn't auto-derive Clone)
+impl Clone for MapBox {
+    fn clone(&self) -> Self {
+        let data = self.data.read().unwrap();
+        let cloned_data: HashMap<String, Box<dyn NyashBox>> = data.iter()
+            .map(|(k, v)| (k.clone(), v.clone_box()))
+            .collect();
+        MapBox {
+            data: RwLock::new(cloned_data),
+            base: BoxBase::new(),
+        }
     }
 }
 
@@ -233,7 +246,7 @@ impl BoxCore for MapBox {
     }
     
     fn fmt_box(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        let size = self.data.lock().unwrap().len();
+        let size = self.data.read().unwrap().len();
         write!(f, "MapBox(size={})", size)
     }
     
@@ -252,7 +265,7 @@ impl NyashBox for MapBox {
     }
     
     fn to_string_box(&self) -> StringBox {
-        let size = self.data.lock().unwrap().len();
+        let size = self.data.read().unwrap().len();
         StringBox::new(&format!("MapBox(size={})", size))
     }
     
@@ -279,7 +292,7 @@ impl Display for MapBox {
 
 impl Debug for MapBox {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let data = self.data.lock().unwrap();
+        let data = self.data.read().unwrap();
         f.debug_struct("MapBox")
             .field("id", &self.base.id)
             .field("size", &data.len())