diff --git a/src/mir/join_ir_ops.rs b/src/mir/join_ir_ops.rs
index 6f052d9b..4c6736aa 100644
--- a/src/mir/join_ir_ops.rs
+++ b/src/mir/join_ir_ops.rs
@@ -271,3 +271,124 @@ mod tests {
         assert!(result.unwrap_err().message.contains("Type mismatch"));
     }
 }
+
+/// Phase 27-shortterm S-4.2: JoinValue ↔ VMValue conversion
+///
+/// Provides bidirectional conversion between JoinIR's lightweight JoinValue
+/// and VM's runtime VMValue representation.
+///
+/// ## Conversion Strategy
+/// - JoinValue::Int    ↔ VMValue::Integer
+/// - JoinValue::Bool   ↔ VMValue::Bool
+/// - JoinValue::Str    ↔ VMValue::String
+/// - JoinValue::Unit   ↔ VMValue::Void
+/// - VMValue::Float    → Error (not supported in JoinValue)
+/// - VMValue::Future   → Error (not supported in JoinValue)
+/// - VMValue::BoxRef   → Error (requires NyashBox downcast)
+impl JoinValue {
+    /// Convert JoinValue to VMValue for VM execution
+    ///
+    /// # Example
+    /// ```ignore
+    /// let join_val = JoinValue::Int(42);
+    /// let vm_val = join_val.to_vm_value();
+    /// assert!(matches!(vm_val, VMValue::Integer(42)));
+    /// ```
+    pub fn to_vm_value(&self) -> crate::backend::VMValue {
+        match self {
+            JoinValue::Int(i) => crate::backend::VMValue::Integer(*i),
+            JoinValue::Bool(b) => crate::backend::VMValue::Bool(*b),
+            JoinValue::Str(s) => crate::backend::VMValue::String(s.clone()),
+            JoinValue::Unit => crate::backend::VMValue::Void,
+        }
+    }
+
+    /// Convert VMValue to JoinValue
+    ///
+    /// Returns error for VMValue types not representable in JoinValue
+    /// (Float, Future, BoxRef).
+    ///
+    /// # Example
+    /// ```ignore
+    /// let vm_val = VMValue::Integer(42);
+    /// let join_val = JoinValue::from_vm_value(&vm_val)?;
+    /// assert_eq!(join_val, JoinValue::Int(42));
+    /// ```
+    pub fn from_vm_value(vm_val: &crate::backend::VMValue) -> Result<Self, JoinIrOpError> {
+        match vm_val {
+            crate::backend::VMValue::Integer(i) => Ok(JoinValue::Int(*i)),
+            crate::backend::VMValue::Bool(b) => Ok(JoinValue::Bool(*b)),
+            crate::backend::VMValue::String(s) => Ok(JoinValue::Str(s.clone())),
+            crate::backend::VMValue::Void => Ok(JoinValue::Unit),
+            crate::backend::VMValue::Float(_) => {
+                Err(JoinIrOpError::new("Float not supported in JoinValue"))
+            }
+            crate::backend::VMValue::Future(_) => {
+                Err(JoinIrOpError::new("Future not supported in JoinValue"))
+            }
+            crate::backend::VMValue::BoxRef(_) => {
+                Err(JoinIrOpError::new("BoxRef not directly convertible to JoinValue - requires NyashBox downcast"))
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod vm_conversion_tests {
+    use super::*;
+
+    #[test]
+    fn test_joinvalue_to_vmvalue() {
+        // Int → Integer
+        let join_int = JoinValue::Int(42);
+        let vm_val = join_int.to_vm_value();
+        assert!(matches!(vm_val, crate::backend::VMValue::Integer(42)));
+
+        // Bool → Bool
+        let join_bool = JoinValue::Bool(true);
+        let vm_val = join_bool.to_vm_value();
+        assert!(matches!(vm_val, crate::backend::VMValue::Bool(true)));
+
+        // Str → String
+        let join_str = JoinValue::Str("hello".to_string());
+        let vm_val = join_str.to_vm_value();
+        assert!(matches!(vm_val, crate::backend::VMValue::String(s) if s == "hello"));
+
+        // Unit → Void
+        let join_unit = JoinValue::Unit;
+        let vm_val = join_unit.to_vm_value();
+        assert!(matches!(vm_val, crate::backend::VMValue::Void));
+    }
+
+    #[test]
+    fn test_vmvalue_to_joinvalue() {
+        // Integer → Int
+        let vm_int = crate::backend::VMValue::Integer(42);
+        let join_val = JoinValue::from_vm_value(&vm_int).unwrap();
+        assert_eq!(join_val, JoinValue::Int(42));
+
+        // Bool → Bool
+        let vm_bool = crate::backend::VMValue::Bool(false);
+        let join_val = JoinValue::from_vm_value(&vm_bool).unwrap();
+        assert_eq!(join_val, JoinValue::Bool(false));
+
+        // String → Str
+        let vm_str = crate::backend::VMValue::String("world".to_string());
+        let join_val = JoinValue::from_vm_value(&vm_str).unwrap();
+        assert_eq!(join_val, JoinValue::Str("world".to_string()));
+
+        // Void → Unit
+        let vm_void = crate::backend::VMValue::Void;
+        let join_val = JoinValue::from_vm_value(&vm_void).unwrap();
+        assert_eq!(join_val, JoinValue::Unit);
+    }
+
+    #[test]
+    fn test_vmvalue_to_joinvalue_unsupported() {
+        // Float → Error
+        let vm_float = crate::backend::VMValue::Float(3.14);
+        let result = JoinValue::from_vm_value(&vm_float);
+        assert!(result.is_err());
+        assert!(result.unwrap_err().message.contains("Float not supported"));
+    }
+}
diff --git a/src/mir/join_ir_vm_bridge.rs b/src/mir/join_ir_vm_bridge.rs
new file mode 100644
index 00000000..fcfba1c3
--- /dev/null
+++ b/src/mir/join_ir_vm_bridge.rs
@@ -0,0 +1,361 @@
+//! Phase 27-shortterm S-4: JoinIR → Rust VM Bridge
+//!
+//! 目的: JoinIR（正規化された IR）を Rust VM で実行するブリッジ層
+//!
+//! ## Architecture
+//! ```text
+//! JoinIR (normalized) → MirModule → Rust VM → Result
+//!       ↑                 ↑          ↑
+//!   PHI bugs         VM input    Execution
+//!   eliminated         format    (GC, plugins)
+//! ```
+//!
+//! ## Design Principles
+//! - JoinIR の正規化構造を保持したまま VM に渡す
+//! - マッピングだけで済ませる（JoinIR でやった正規化は消えない）
+//! - VM の機能（GC、プラグイン、エラーハンドリング）を活用
+//!
+//! ## Minimal Instruction Set (S-4.3)
+//! - **Compute**: Const, BinOp, Compare
+//! - **BoxCall**: StringBox メソッド呼び出し
+//! - **Call/Jump/Ret**: 制御フロー
+//!
+//! Phase 27-shortterm scope: skip_ws で green 化できれば成功
+
+use crate::backend::{MirInterpreter, VMError, VMValue};
+use crate::mir::join_ir::{
+    BinOpKind, CompareOp, ConstValue, JoinFuncId, JoinInst, JoinModule, MirLikeInst,
+};
+use crate::mir::join_ir_ops::JoinValue;
+use crate::mir::{
+    BasicBlockId, BinaryOp, CompareOp as MirCompareOp, ConstValue as MirConstValue,
+    EffectMask, FunctionSignature, MirFunction, MirInstruction, MirModule, MirType, ValueId,
+};
+use std::collections::HashMap;
+
+/// Phase 27-shortterm S-4 エラー型
+#[derive(Debug, Clone)]
+pub struct JoinIrVmBridgeError {
+    pub message: String,
+}
+
+impl JoinIrVmBridgeError {
+    pub fn new(msg: impl Into<String>) -> Self {
+        Self {
+            message: msg.into(),
+        }
+    }
+}
+
+impl From<VMError> for JoinIrVmBridgeError {
+    fn from(err: VMError) -> Self {
+        JoinIrVmBridgeError::new(format!("VM error: {:?}", err))
+    }
+}
+
+/// Phase 27-shortterm S-4.3: JoinIR → VM 実行のエントリーポイント
+///
+/// ## Arguments
+/// - `join_module`: JoinIR モジュール（正規化済み）
+/// - `entry_func`: エントリーポイント関数ID
+/// - `args`: 初期引数（JoinValue 形式）
+///
+/// ## Returns
+/// - `Ok(JoinValue)`: 実行結果
+/// - `Err(JoinIrVmBridgeError)`: 変換エラーまたは実行エラー
+///
+/// ## Example
+/// ```ignore
+/// let join_module = lower_skip_ws_to_joinir(&mir_module)?;
+/// let result = run_joinir_via_vm(
+///     &join_module,
+///     JoinFuncId::new(0),
+///     &[JoinValue::Str("  hello".to_string()), JoinValue::Int(7)]
+/// )?;
+/// assert_eq!(result, JoinValue::Int(2));
+/// ```
+pub fn run_joinir_via_vm(
+    join_module: &JoinModule,
+    entry_func: JoinFuncId,
+    args: &[JoinValue],
+) -> Result<JoinValue, JoinIrVmBridgeError> {
+    eprintln!("[joinir_vm_bridge] Phase 27-shortterm S-4.3");
+    eprintln!(
+        "[joinir_vm_bridge] Converting JoinIR to MIR for VM execution"
+    );
+
+    // Step 1: JoinIR → MIR 変換
+    let mir_module = convert_joinir_to_mir(join_module, entry_func)?;
+
+    eprintln!(
+        "[joinir_vm_bridge] Converted {} JoinIR functions to MIR",
+        join_module.functions.len()
+    );
+
+    // Step 2: VM 実行
+    let mut vm = MirInterpreter::new();
+
+    // 初期引数を VM に設定（暫定実装: 環境変数経由）
+    // TODO: S-4.4 で引数渡し機構を追加
+    eprintln!(
+        "[joinir_vm_bridge] Executing via VM with {} arguments",
+        args.len()
+    );
+
+    let result_box = vm.execute_module(&mir_module)?;
+
+    // Step 3: VMValue → JoinValue 変換
+    let vm_value = VMValue::from_nyash_box(result_box);
+    let join_result = JoinValue::from_vm_value(&vm_value)
+        .map_err(|e| JoinIrVmBridgeError::new(format!("Result conversion error: {}", e.message)))?;
+
+    eprintln!("[joinir_vm_bridge] Execution succeeded: {:?}", join_result);
+
+    Ok(join_result)
+}
+
+/// Phase 27-shortterm S-4.3: JoinIR → MIR 変換器
+///
+/// JoinIR の関数群を MIR モジュールに変換する。
+/// JoinIR の正規化構造（Pinned/Carrier、Exit φ）は関数引数として表現されているため、
+/// MIR に変換しても構造的意味は保持される。
+fn convert_joinir_to_mir(
+    join_module: &JoinModule,
+    entry_func: JoinFuncId,
+) -> Result<MirModule, JoinIrVmBridgeError> {
+    let mut mir_module = MirModule::new("joinir_bridge".to_string());
+
+    // すべての JoinIR 関数を MIR 関数に変換
+    for (func_id, join_func) in &join_module.functions {
+        eprintln!(
+            "[joinir_vm_bridge] Converting JoinFunction {} ({})",
+            func_id.0, join_func.name
+        );
+
+        let mir_func = convert_join_function_to_mir(join_func)?;
+
+        // Entry function を "main" として登録
+        let func_name = if *func_id == entry_func {
+            "main".to_string()
+        } else {
+            format!("join_func_{}", func_id.0)
+        };
+
+        mir_module.functions.insert(func_name, mir_func);
+    }
+
+    Ok(mir_module)
+}
+
+/// JoinFunction → MirFunction 変換
+fn convert_join_function_to_mir(
+    join_func: &crate::mir::join_ir::JoinFunction,
+) -> Result<MirFunction, JoinIrVmBridgeError> {
+    // Phase 27-shortterm S-4.3 最小実装:
+    // 1つの basic block に全命令を配置
+    let entry_block = BasicBlockId(0);
+
+    // Create minimal FunctionSignature for JoinIR function
+    // Phase 27-shortterm: すべて MirType::Unknown として扱う（型情報は JoinIR に無いため）
+    let param_types = join_func.params.iter()
+        .map(|_| MirType::Unknown)
+        .collect::<Vec<_>>();
+
+    let signature = FunctionSignature {
+        name: join_func.name.clone(),
+        params: param_types,
+        return_type: MirType::Unknown,
+        effects: EffectMask::PURE,
+    };
+
+    let mut mir_func = MirFunction::new(signature, entry_block);
+
+    let mut instructions = Vec::new();
+
+    // JoinInst → MirInstruction 変換
+    for join_inst in &join_func.body {
+        match join_inst {
+            JoinInst::Compute(mir_like) => {
+                let mir_inst = convert_mir_like_inst(mir_like)?;
+                instructions.push(mir_inst);
+            }
+            JoinInst::Call { func, args, dst, .. } => {
+                // TODO: S-4.4 で関数呼び出し実装
+                eprintln!(
+                    "[joinir_vm_bridge] WARNING: Call instruction not yet implemented (func={:?}, args={:?}, dst={:?})",
+                    func, args, dst
+                );
+            }
+            JoinInst::Jump { cont, args, cond } => {
+                // TODO: S-4.4 で継続実装
+                eprintln!(
+                    "[joinir_vm_bridge] WARNING: Jump instruction not yet implemented (cont={:?}, args={:?}, cond={:?})",
+                    cont, args, cond
+                );
+            }
+            JoinInst::Ret { value } => {
+                if let Some(val_id) = value {
+                    instructions.push(MirInstruction::Return {
+                        value: Some(*val_id),
+                    });
+                } else {
+                    instructions.push(MirInstruction::Return { value: None });
+                }
+            }
+        }
+    }
+
+    // Entry block に命令を登録
+    if let Some(block) = mir_func.blocks.get_mut(&entry_block) {
+        block.instructions = instructions;
+    }
+
+    Ok(mir_func)
+}
+
+/// MirLikeInst → MirInstruction 変換
+fn convert_mir_like_inst(
+    mir_like: &MirLikeInst,
+) -> Result<MirInstruction, JoinIrVmBridgeError> {
+    match mir_like {
+        MirLikeInst::Const { dst, value } => {
+            let mir_const = match value {
+                ConstValue::Integer(i) => MirConstValue::Integer(*i),
+                ConstValue::Bool(b) => MirConstValue::Bool(*b),
+                ConstValue::String(s) => MirConstValue::String(s.clone()),
+                ConstValue::Null => MirConstValue::Null,
+            };
+            Ok(MirInstruction::Const {
+                dst: *dst,
+                value: mir_const,
+            })
+        }
+        MirLikeInst::BinOp { dst, op, lhs, rhs } => {
+            let mir_op = match op {
+                BinOpKind::Add => BinaryOp::Add,
+                BinOpKind::Sub => BinaryOp::Sub,
+                BinOpKind::Mul => BinaryOp::Mul,
+                BinOpKind::Div => BinaryOp::Div,
+                BinOpKind::Or => BinaryOp::Or,
+                BinOpKind::And => BinaryOp::And,
+            };
+            Ok(MirInstruction::BinOp {
+                dst: *dst,
+                op: mir_op,
+                lhs: *lhs,
+                rhs: *rhs,
+            })
+        }
+        MirLikeInst::Compare { dst, op, lhs, rhs } => {
+            let mir_cmp = match op {
+                CompareOp::Lt => MirCompareOp::Lt,
+                CompareOp::Le => MirCompareOp::Le,
+                CompareOp::Gt => MirCompareOp::Gt,
+                CompareOp::Ge => MirCompareOp::Ge,
+                CompareOp::Eq => MirCompareOp::Eq,
+                CompareOp::Ne => MirCompareOp::Ne,
+            };
+            Ok(MirInstruction::Compare {
+                dst: *dst,
+                op: mir_cmp,
+                lhs: *lhs,
+                rhs: *rhs,
+            })
+        }
+        MirLikeInst::BoxCall {
+            dst,
+            box_name,
+            method,
+            args,
+        } => {
+            // Phase 27-shortterm S-4.3: BoxCall → MIR BoxCall
+            // box_name は JoinIR で保持しているが、MIR BoxCall には receiver ValueId のみ
+            // 暫定: args[0] を receiver として扱う
+            if args.is_empty() {
+                return Err(JoinIrVmBridgeError::new(format!(
+                    "BoxCall requires at least one argument (receiver), got: box_name={}, method={}",
+                    box_name, method
+                )));
+            }
+
+            let receiver = args[0];
+            let method_args = args[1..].to_vec();
+
+            Ok(MirInstruction::BoxCall {
+                dst: *dst,
+                box_val: receiver,
+                method: method.clone(),
+                method_id: None, // Phase 27-shortterm: no method ID resolution
+                args: method_args,
+                effects: EffectMask::PURE, // Phase 27-shortterm: assume pure
+            })
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_convert_const_inst() {
+        let join_const = MirLikeInst::Const {
+            dst: ValueId(10),
+            value: ConstValue::Integer(42),
+        };
+
+        let mir_inst = convert_mir_like_inst(&join_const).unwrap();
+
+        match mir_inst {
+            MirInstruction::Const { dst, value } => {
+                assert_eq!(dst, ValueId(10));
+                assert!(matches!(value, MirConstValue::Integer(42)));
+            }
+            _ => panic!("Expected Const instruction"),
+        }
+    }
+
+    #[test]
+    fn test_convert_binop_inst() {
+        let join_binop = MirLikeInst::BinOp {
+            dst: ValueId(20),
+            op: BinOpKind::Add,
+            lhs: ValueId(10),
+            rhs: ValueId(11),
+        };
+
+        let mir_inst = convert_mir_like_inst(&join_binop).unwrap();
+
+        match mir_inst {
+            MirInstruction::BinOp { dst, op, lhs, rhs } => {
+                assert_eq!(dst, ValueId(20));
+                assert_eq!(op, BinaryOp::Add);
+                assert_eq!(lhs, ValueId(10));
+                assert_eq!(rhs, ValueId(11));
+            }
+            _ => panic!("Expected BinOp instruction"),
+        }
+    }
+
+    #[test]
+    fn test_convert_compare_inst() {
+        let join_cmp = MirLikeInst::Compare {
+            dst: ValueId(30),
+            op: CompareOp::Ge,
+            lhs: ValueId(10),
+            rhs: ValueId(11),
+        };
+
+        let mir_inst = convert_mir_like_inst(&join_cmp).unwrap();
+
+        match mir_inst {
+            MirInstruction::Compare { dst, op, lhs, rhs } => {
+                assert_eq!(dst, ValueId(30));
+                assert_eq!(op, MirCompareOp::Ge);
+                assert_eq!(lhs, ValueId(10));
+                assert_eq!(rhs, ValueId(11));
+            }
+            _ => panic!("Expected Compare instruction"),
+        }
+    }
+}
diff --git a/src/mir/mod.rs b/src/mir/mod.rs
index 4f9f522e..b34e3dcb 100644
--- a/src/mir/mod.rs
+++ b/src/mir/mod.rs
@@ -40,6 +40,7 @@ pub mod query; // Phase 26-G: MIR read/write/CFGビュー (MirQuery)
 pub mod join_ir; // Phase 26-H: 関数正規化IR（JoinIR）
 pub mod join_ir_runner; // Phase 27.2: JoinIR 実行器（実験用）
 pub mod join_ir_ops; // Phase 27.8: JoinIR 命令意味箱（ops box）
+pub mod join_ir_vm_bridge; // Phase 27-shortterm S-4: JoinIR → Rust VM ブリッジ
 pub mod verification;
 pub mod verification_types; // extracted error types // Optimization subpasses (e.g., type_hints) // Phase 25.1f: Loop/If 共通ビュー（ControlForm）