diff --git a/src/mir/join_ir/frontend/ast_lowerer/expr.rs b/src/mir/join_ir/frontend/ast_lowerer/expr.rs
index b91e3b16..46d9a6b1 100644
--- a/src/mir/join_ir/frontend/ast_lowerer/expr.rs
+++ b/src/mir/join_ir/frontend/ast_lowerer/expr.rs
@@ -234,11 +234,16 @@ impl AstToJoinIrLowerer {
                 // 結果変数を割り当て
                 let dst = ctx.alloc_var();
 
+                // Phase 65-2-B: Box 名から型ヒントを推論
+                let type_hint =
+                    crate::mir::join_ir::lowering::type_inference::infer_box_type(box_name);
+
                 // NewBox 命令を生成
                 let newbox_inst = JoinInst::NewBox {
                     dst,
                     box_name: box_name.to_string(),
                     args: arg_vars,
+                    type_hint, // Phase 65-2-B: P3-B Box コンストラクタ型ヒント
                 };
 
                 let mut insts = arg_insts;
diff --git a/src/mir/join_ir/json.rs b/src/mir/join_ir/json.rs
index b4042bff..17cde1a9 100644
--- a/src/mir/join_ir/json.rs
+++ b/src/mir/join_ir/json.rs
@@ -277,6 +277,7 @@ fn write_inst<W: Write>(inst: &JoinInst, out: &mut W) -> std::io::Result<()> {
             dst,
             box_name,
             args,
+            type_hint, // Phase 65-2-B: 型ヒント追加（JSON には現状出力しない）
         } => {
             write!(out, "{{\"type\":\"new_box\"")?;
             write!(out, ",\"dst\":{}", dst.0)?;
@@ -289,6 +290,8 @@ fn write_inst<W: Write>(inst: &JoinInst, out: &mut W) -> std::io::Result<()> {
                 write!(out, "{}", arg.0)?;
             }
             write!(out, "]")?;
+            // Phase 65-2-B: TODO: type_hint を JSON に含めるかは Phase 65-3 で検討
+            let _ = type_hint; // unused warning 回避
             write!(out, "}}")?;
         }
         JoinInst::Compute(mir_like) => {
diff --git a/src/mir/join_ir/mod.rs b/src/mir/join_ir/mod.rs
index 8ef1554e..0120065b 100644
--- a/src/mir/join_ir/mod.rs
+++ b/src/mir/join_ir/mod.rs
@@ -359,10 +359,14 @@ pub enum JoinInst {
     /// Phase 51: Box インスタンス生成
     /// new BoxName(args...) の構造を JoinIR で表現
     /// MIR 変換時に NewBox 命令に変換
+    ///
+    /// Phase 65-2-B: type_hint で生成される Box 型を伝播（infer_type_from_phi 削減用）
     NewBox {
         dst: VarId,
         box_name: String,
         args: Vec<VarId>,
+        /// Phase 65-2-B: 生成される Box の型ヒント（P3-B Box コンストラクタ対応）
+        type_hint: Option<MirType>,
     },
 
     /// Phase 41-4: 深いネスト if の複数変数 merge（else なし）
diff --git a/src/mir/join_ir_vm_bridge/convert.rs b/src/mir/join_ir_vm_bridge/convert.rs
index ff4eff5b..aa5ebd1a 100644
--- a/src/mir/join_ir_vm_bridge/convert.rs
+++ b/src/mir/join_ir_vm_bridge/convert.rs
@@ -323,6 +323,7 @@ pub(crate) fn convert_join_function_to_mir(
                 dst,
                 box_name,
                 args,
+                type_hint, // Phase 65-2-B: 型ヒント追加（現状は無視、Phase 65-3 で活用）
             } => {
                 let mir_inst = MirInstruction::NewBox {
                     dst: *dst,
@@ -330,6 +331,9 @@ pub(crate) fn convert_join_function_to_mir(
                     args: args.clone(),
                 };
                 current_instructions.push(mir_inst);
+
+                // Phase 65-2-B: TODO: type_hint を value_types に記録する処理を Phase 65-3 で追加
+                let _ = type_hint; // 現状は unused warning 回避
             }
             JoinInst::Call {
                 func,