diff --git a/src/boxes/array/mod.rs b/src/boxes/array/mod.rs
index 63842a9b..4065f09f 100644
--- a/src/boxes/array/mod.rs
+++ b/src/boxes/array/mod.rs
@@ -4,11 +4,11 @@
 
 use crate::box_trait::{NyashBox, StringBox, BoolBox, IntegerBox, BoxCore, BoxBase};
 use std::any::Any;
-use std::sync::RwLock;
+use std::sync::{Arc, RwLock};
 use std::fmt::Display;
 
 pub struct ArrayBox {
-    pub items: RwLock<Vec<Box<dyn NyashBox>>>,
+    pub items: Arc<RwLock<Vec<Box<dyn NyashBox>>>>,  // Arc追加
     base: BoxBase,
 }
 
@@ -16,7 +16,7 @@ impl ArrayBox {
     /// 新しいArrayBoxを作成
     pub fn new() -> Self {
         ArrayBox { 
-            items: RwLock::new(Vec::new()),
+            items: Arc::new(RwLock::new(Vec::new())),  // Arc::new追加
             base: BoxBase::new(),
         }
     }
@@ -24,7 +24,7 @@ impl ArrayBox {
     /// 要素を持つArrayBoxを作成
     pub fn new_with_elements(elements: Vec<Box<dyn NyashBox>>) -> Self {
         ArrayBox { 
-            items: RwLock::new(elements),
+            items: Arc::new(RwLock::new(elements)),    // Arc::new追加
             base: BoxBase::new(),
         }
     }
@@ -238,11 +238,16 @@ 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())
+        // ディープコピー（独立インスタンス）
+        let items_guard = self.items.read().unwrap();
+        let cloned_items: Vec<Box<dyn NyashBox>> = items_guard.iter()
+            .map(|item| item.clone_box())  // 要素もディープコピー
             .collect();
-        ArrayBox::new_with_elements(cloned_items)
+        
+        ArrayBox {
+            items: Arc::new(RwLock::new(cloned_items)),  // 新しいArc
+            base: BoxBase::new(),
+        }
     }
 }
 
@@ -283,9 +288,13 @@ impl NyashBox for ArrayBox {
         Box::new(self.clone())
     }
     
-    /// 仮実装: clone_boxと同じ（後で修正）
+    /// 🎯 状態共有の核心実装
     fn share_box(&self) -> Box<dyn NyashBox> {
-        self.clone_box()
+        let new_instance = ArrayBox {
+            items: Arc::clone(&self.items),  // Arcクローンで状態共有
+            base: BoxBase::new(),            // 新しいID
+        };
+        Box::new(new_instance)
     }
 
     fn to_string_box(&self) -> StringBox {
diff --git a/src/interpreter/expressions.rs b/src/interpreter/expressions.rs
index f1cb46be..fb0e87db 100644
--- a/src/interpreter/expressions.rs
+++ b/src/interpreter/expressions.rs
@@ -105,7 +105,7 @@ impl NyashInterpreter {
                         name: name.clone(), 
                         span: expression.span() 
                     })?;
-                Ok((*shared_var).clone_box())  // Convert for external interface
+                Ok((*shared_var).share_box())  // 🎯 State-sharing instead of cloning
             }
             
             ASTNode::BinaryOp { operator, left, right, .. } => {
diff --git a/test_phase_b_validation.nyash b/test_phase_b_validation.nyash
new file mode 100644
index 00000000..60d61e47
--- /dev/null
+++ b/test_phase_b_validation.nyash
@@ -0,0 +1,24 @@
+// 🚨 ArrayBox状態保持テスト - コアとなる問題を検証
+static box Main {
+    init { result1, result2, result3 }
+    main() {
+        // テスト1: 基本的な状態保持
+        local arr1
+        arr1 = new ArrayBox()
+        arr1.push("hello")
+        me.result1 = arr1.length()  // 期待値: 1
+
+        // テスト2: 複数操作の状態保持
+        local arr2
+        arr2 = new ArrayBox()
+        arr2.push("first")
+        arr2.push("second")
+        me.result2 = arr2.length()  // 期待値: 2
+        
+        // テスト3: 変数再利用での状態保持
+        arr1.push("world")
+        me.result3 = arr1.length()  // 期待値: 2
+        
+        return me.result1
+    }
+}
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diff --git a/tests/array_state_sharing_test.rs b/tests/array_state_sharing_test.rs
new file mode 100644
index 00000000..073cd413
--- /dev/null
+++ b/tests/array_state_sharing_test.rs
@@ -0,0 +1,70 @@
+#[cfg(test)]
+mod array_state_sharing_tests {
+    use crate::interpreter::Interpreter;
+    use crate::boxes::array::ArrayBox;
+    use crate::box_trait::{NyashBox, IntegerBox, StringBox};
+
+    #[test]
+    fn test_arraybox_state_sharing_bug_fix() {
+        // 🚨 問題再現テスト
+        let mut interpreter = Interpreter::new();
+        let program = r#"
+            static box Main {
+                init { result }
+                main() {
+                    local arr
+                    arr = new ArrayBox()
+                    arr.push("hello")
+                    me.result = arr.length()
+                    return me.result
+                }
+            }
+        "#;
+        
+        let result = interpreter.execute_program(program).unwrap();
+        let int_result = result.as_any().downcast_ref::<IntegerBox>().unwrap();
+        assert_eq!(int_result.value, 1);  // 🎯 0ではなく1を返すべき
+    }
+
+    #[test]
+    fn test_share_box_vs_clone_box_semantics() {
+        let arr1 = ArrayBox::new();
+        arr1.push(Box::new(StringBox::new("hello")));
+        
+        // share_box: 状態共有
+        let arr2 = arr1.share_box();
+        let arr2_array = arr2.as_any().downcast_ref::<ArrayBox>().unwrap();
+        assert_eq!(arr2_array.len(), 1);  // 共有されている
+        
+        // clone_box: 独立
+        let arr3 = arr1.clone_box();
+        let arr3_array = arr3.as_any().downcast_ref::<ArrayBox>().unwrap();
+        arr1.push(Box::new(StringBox::new("world")));
+        assert_eq!(arr3_array.len(), 1);  // 影響を受けない
+        assert_eq!(arr1.len(), 2);        // 元は2要素
+        assert_eq!(arr2_array.len(), 2);  // 共有されているので2要素
+    }
+
+    #[test]
+    fn test_multiple_operations_state_preservation() {
+        let mut interpreter = Interpreter::new();
+        let program = r#"
+            static box Main {
+                init { result }
+                main() {
+                    local arr
+                    arr = new ArrayBox()
+                    arr.push("first")
+                    arr.push("second")
+                    arr.push("third")
+                    me.result = arr.length()
+                    return me.result
+                }
+            }
+        "#;
+        
+        let result = interpreter.execute_program(program).unwrap();
+        let int_result = result.as_any().downcast_ref::<IntegerBox>().unwrap();
+        assert_eq!(int_result.value, 3);  // 3要素が正しく保持されるべき
+    }
+}
\ No newline at end of file