diff --git a/src/mir/builder/calls/emit.rs b/src/mir/builder/calls/emit.rs
index cb8bbd9a..f1526928 100644
--- a/src/mir/builder/calls/emit.rs
+++ b/src/mir/builder/calls/emit.rs
@@ -107,8 +107,9 @@ impl MirBuilder {
         callee = self.materialize_receiver_in_callee(callee)?;
 
         // Structural guard: prevent static compiler boxes from being called with runtime receivers
-        // If box_kind is StaticCompiler but receiver has a runtime Box type, normalize to runtime
-        callee = self.apply_static_runtime_guard(callee)?;
+        // 箱理論: CalleeGuardBox による構造的分離
+        let guard = super::guard::CalleeGuardBox::new(&self.value_types);
+        callee = guard.apply_static_runtime_guard(callee)?;
 
         // Emit resolve.choose for method callee (dev-only; default OFF)
         if let Callee::Method { box_name, method, certainty, .. } = &callee {
@@ -455,59 +456,6 @@ impl MirBuilder {
         })
     }
 
-    /// Structural guard: prevent static compiler boxes from mixing with runtime data boxes
-    ///
-    /// 箱理論の「境界を作る」原則: Stage-B/Stage-1コンパイラBoxとランタイムDataBoxを
-    /// 構造レベルで分離し、型メタデータの混入を防ぐ。
-    ///
-    /// If box_kind is StaticCompiler but receiver has a runtime Box type (MapBox/ArrayBox/etc.),
-    /// normalize box_name to the runtime type. This prevents cases like:
-    /// - Stage1UsingResolverBox.get where receiver is actually MapBox
-    /// - StageBArgsBox.length where receiver is actually ArrayBox
-    ///
-    /// This is a Fail-Fast structural guard, not a fallback.
-    fn apply_static_runtime_guard(&self, callee: Callee) -> Result<Callee, String> {
-        use crate::mir::definitions::call_unified::CalleeBoxKind;
-
-        if let Callee::Method { ref box_name, ref method, receiver: Some(recv), certainty, box_kind } = callee {
-            // Only apply guard if box_kind is StaticCompiler
-            if box_kind == CalleeBoxKind::StaticCompiler {
-                // Check if receiver has a Box type
-                if let Some(crate::mir::MirType::Box(receiver_box)) = self.value_types.get(&recv) {
-                    let trace_enabled = std::env::var("NYASH_CALLEE_RESOLVE_TRACE").ok().as_deref() == Some("1");
-
-                    // If receiver box type matches the static box name, this is a me-call
-                    // Let it through for static method lowering (don't normalize)
-                    if receiver_box == box_name {
-                        if trace_enabled {
-                            eprintln!("[static-runtime-guard] ME-CALL detected:");
-                            eprintln!("  {}.{} with receiver type: {} (same as box_name)", box_name, method, receiver_box);
-                            eprintln!("  → Allowing for static method lowering");
-                        }
-                        return Ok(callee);  // Pass through unchanged
-                    }
-
-                    // Otherwise, this is a true mix-up: runtime box with static box name
-                    // Normalize to the runtime box type
-                    if trace_enabled {
-                        eprintln!("[static-runtime-guard] CORRECTING mix-up:");
-                        eprintln!("  Original: {}.{} (box_kind=StaticCompiler)", box_name, method);
-                        eprintln!("  Receiver %{} has runtime type: {}", recv.0, receiver_box);
-                        eprintln!("  Normalized: {}.{}", receiver_box, method);
-                    }
-
-                    return Ok(Callee::Method {
-                        box_name: receiver_box.clone(),
-                        method: method.clone(),
-                        receiver: Some(recv),
-                        certainty,
-                        box_kind: CalleeBoxKind::RuntimeData,  // Switch to runtime
-                    });
-                }
-            }
-        }
-
-        // No guard needed, return as-is
-        Ok(callee)
-    }
+    // ✅ 箱化完了: apply_static_runtime_guard → CalleeGuardBox::apply_static_runtime_guard
+    // 構造ガードロジックは src/mir/builder/calls/guard.rs に移動済み
 }
diff --git a/src/mir/builder/calls/guard.rs b/src/mir/builder/calls/guard.rs
new file mode 100644
index 00000000..956e684f
--- /dev/null
+++ b/src/mir/builder/calls/guard.rs
@@ -0,0 +1,195 @@
+/*!
+ * CalleeGuardBox - 構造ガード専用箱
+ *
+ * 箱理論の実践:
+ * - 箱にする: 構造ガード機能を1箱に集約
+ * - 境界を作る: 静的Box/ランタイムBoxの混線を構造的に防ぐ
+ * - Fail-Fast: フォールバックより明示的エラー
+ *
+ * 責務:
+ * - Callee::Methodの静的Box/ランタイムBox混線検出・正規化
+ * - me-call判定（receiver型==box_name）
+ * - receiver実体化の保証
+ */
+
+use crate::mir::{Callee, MirType, ValueId};
+use crate::mir::definitions::call_unified::CalleeBoxKind;
+use std::collections::HashMap;
+
+/// 構造ガード専用箱
+///
+/// 箱理論:
+/// - 単一責務: Calleeの構造検証・正規化のみ
+/// - 状態最小: value_typesのみ保持（型情報参照用）
+/// - ピュア関数的: 入力Callee → 検証・変換 → 出力Callee
+pub struct CalleeGuardBox<'a> {
+    /// 型情報マップ（ValueId → MirType）
+    value_types: &'a HashMap<ValueId, MirType>,
+}
+
+impl<'a> CalleeGuardBox<'a> {
+    /// 新しいCalleeGuardBoxを作成
+    pub fn new(value_types: &'a HashMap<ValueId, MirType>) -> Self {
+        CalleeGuardBox { value_types }
+    }
+
+    /// 静的Box/ランタイムBox混線を検出・正規化
+    ///
+    /// 箱理論の「境界を作る」原則:
+    /// - Stage-B/Stage-1コンパイラBoxとランタイムDataBoxを構造的に分離
+    ///
+    /// ロジック:
+    /// 1. box_kind==StaticCompiler かつ receiver型==同一Box名
+    ///    → me-call判定、静的メソッド降下に委ねる（そのまま通す）
+    /// 2. box_kind==StaticCompiler かつ receiver型==異なるランタイムBox
+    ///    → 正規化（MapBox/ArrayBoxなど実際のruntime型に修正）
+    /// 3. それ以外 → そのまま通す
+    ///
+    /// 実例:
+    /// - StageBArgsBox.resolve_src内のargs.get(i)がStage1UsingResolverBox.getに
+    ///   化けるのを防ぐ（args型はMapBox/ArrayBox → 正規化）
+    pub fn apply_static_runtime_guard(&self, callee: Callee) -> Result<Callee, String> {
+        if let Callee::Method { ref box_name, ref method, receiver: Some(recv), certainty, box_kind } = callee {
+            // Only apply guard if box_kind is StaticCompiler
+            if box_kind == CalleeBoxKind::StaticCompiler {
+                // Check if receiver has a Box type
+                if let Some(MirType::Box(receiver_box)) = self.value_types.get(&recv) {
+                    let trace_enabled = std::env::var("NYASH_CALLEE_RESOLVE_TRACE")
+                        .ok()
+                        .as_deref() == Some("1");
+
+                    // If receiver box type matches the static box name, this is a me-call
+                    // Let it through for static method lowering (don't normalize)
+                    if receiver_box == box_name {
+                        if trace_enabled {
+                            eprintln!("[static-runtime-guard] ME-CALL detected:");
+                            eprintln!("  {}.{} with receiver type: {} (same as box_name)", box_name, method, receiver_box);
+                            eprintln!("  → Allowing for static method lowering");
+                        }
+                        return Ok(callee);  // Pass through unchanged
+                    }
+
+                    // Otherwise, this is a true mix-up: runtime box with static box name
+                    // Normalize to the runtime box type
+                    if trace_enabled {
+                        eprintln!("[static-runtime-guard] CORRECTING mix-up:");
+                        eprintln!("  Original: {}.{} (box_kind=StaticCompiler)", box_name, method);
+                        eprintln!("  Receiver %{} has runtime type: {}", recv.0, receiver_box);
+                        eprintln!("  Normalized: {}.{}", receiver_box, method);
+                    }
+
+                    return Ok(Callee::Method {
+                        box_name: receiver_box.clone(),
+                        method: method.clone(),
+                        receiver: Some(recv),
+                        certainty,
+                        box_kind: CalleeBoxKind::RuntimeData,  // Switch to runtime
+                    });
+                }
+            }
+        }
+
+        // No guard needed, return as-is
+        Ok(callee)
+    }
+
+    /// receiver型の検証（ヘルパー）
+    ///
+    /// 指定されたreceiverがBox型を持っているか確認
+    pub fn has_box_type(&self, receiver: ValueId) -> bool {
+        matches!(self.value_types.get(&receiver), Some(MirType::Box(_)))
+    }
+
+    /// receiver型の取得（ヘルパー）
+    ///
+    /// 指定されたreceiverのBox型名を返す（存在しない場合はNone）
+    pub fn get_box_type(&self, receiver: ValueId) -> Option<&String> {
+        match self.value_types.get(&receiver) {
+            Some(MirType::Box(box_name)) => Some(box_name),
+            _ => None,
+        }
+    }
+
+    /// me-call判定
+    ///
+    /// box_name と receiver型が一致するか判定
+    /// （静的メソッド呼び出しの検出用）
+    pub fn is_me_call(&self, box_name: &str, receiver: ValueId) -> bool {
+        match self.get_box_type(receiver) {
+            Some(recv_box) => recv_box == box_name,
+            None => false,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_me_call_detection() {
+        let mut value_types = HashMap::new();
+        value_types.insert(ValueId::new(10), MirType::Box("StageBArgsBox".to_string()));
+
+        let guard = CalleeGuardBox::new(&value_types);
+
+        // Same box name → me-call
+        assert!(guard.is_me_call("StageBArgsBox", ValueId::new(10)));
+
+        // Different box name → not me-call
+        assert!(!guard.is_me_call("Stage1UsingResolverBox", ValueId::new(10)));
+
+        // No type info → not me-call
+        assert!(!guard.is_me_call("StageBArgsBox", ValueId::new(999)));
+    }
+
+    #[test]
+    fn test_static_runtime_guard_me_call() {
+        use crate::mir::definitions::call_unified::TypeCertainty;
+
+        let mut value_types = HashMap::new();
+        value_types.insert(ValueId::new(10), MirType::Box("StageBArgsBox".to_string()));
+
+        let guard = CalleeGuardBox::new(&value_types);
+
+        let callee = Callee::Method {
+            box_name: "StageBArgsBox".to_string(),
+            method: "process".to_string(),
+            receiver: Some(ValueId::new(10)),
+            certainty: TypeCertainty::Known,
+            box_kind: CalleeBoxKind::StaticCompiler,
+        };
+
+        // me-call → should pass through unchanged
+        let result = guard.apply_static_runtime_guard(callee.clone()).unwrap();
+        assert_eq!(result, callee);
+    }
+
+    #[test]
+    fn test_static_runtime_guard_normalization() {
+        use crate::mir::definitions::call_unified::TypeCertainty;
+
+        let mut value_types = HashMap::new();
+        value_types.insert(ValueId::new(10), MirType::Box("MapBox".to_string()));
+
+        let guard = CalleeGuardBox::new(&value_types);
+
+        let callee = Callee::Method {
+            box_name: "Stage1UsingResolverBox".to_string(),
+            method: "get".to_string(),
+            receiver: Some(ValueId::new(10)),
+            certainty: TypeCertainty::Known,
+            box_kind: CalleeBoxKind::StaticCompiler,
+        };
+
+        // Mix-up → should normalize to MapBox
+        let result = guard.apply_static_runtime_guard(callee).unwrap();
+        match result {
+            Callee::Method { box_name, box_kind, .. } => {
+                assert_eq!(box_name, "MapBox");
+                assert_eq!(box_kind, CalleeBoxKind::RuntimeData);
+            }
+            _ => panic!("Expected Method callee"),
+        }
+    }
+}
diff --git a/src/mir/builder/calls/mod.rs b/src/mir/builder/calls/mod.rs
index 35d073ac..1d961f86 100644
--- a/src/mir/builder/calls/mod.rs
+++ b/src/mir/builder/calls/mod.rs
@@ -5,6 +5,7 @@
 //! - utils: ユーティリティ（resolve/parse/extract）
 //! - emit: Call命令発行（統一Call/Legacy Call） ✅ Phase 2完了
 //! - build: Call構築（function call/method call） ✅ Phase 2完了
+//! - guard: 構造ガード（静的Box/ランタイムBox混線防止） ✅ Phase 25.1d完了
 
 // Existing modules (already implemented elsewhere)
 pub mod annotation;
@@ -15,11 +16,12 @@ pub mod function_lowering;
 pub mod method_resolution;
 pub mod special_handlers;
 
-// New refactored modules (Box Theory Phase 1 & 2)
+// New refactored modules (Box Theory Phase 1 & 2 & 25.1d)
 pub mod lowering;
 pub mod utils;
 pub mod emit;    // Phase 2: Call emission
 pub mod build;   // Phase 2: Call building
+pub mod guard;   // Phase 25.1d: Structural guard (static/runtime box separation)
 
 // Re-export public interfaces
 pub use call_target::CallTarget;