diff --git a/docs/private b/docs/private
index 6932ae78..a5a00fc7 160000
--- a/docs/private
+++ b/docs/private
@@ -1 +1 @@
-Subproject commit 6932ae78573177d67ea1266199bf089da4ff3f5c
+Subproject commit a5a00fc717f1f0ee348a867a6d873072400070b4
diff --git a/src/mir/join_ir/lowering/loop_to_join.rs b/src/mir/join_ir/lowering/loop_to_join.rs
new file mode 100644
index 00000000..24c3f481
--- /dev/null
+++ b/src/mir/join_ir/lowering/loop_to_join.rs
@@ -0,0 +1,203 @@
+//! Phase 31: LoopToJoinLowerer - 統一 Loop→JoinIR 変換箱
+//!
+//! このモジュールは MIR の LoopForm を JoinIR に変換する統一インターフェースを提供する。
+//!
+//! ## 設計思想
+//!
+//! - **単一エントリポイント**: `LoopToJoinLowerer::lower()` ですべてのループを処理
+//! - **パターン自動判定**: LoopScopeShape を解析して適切な変換を選択
+//! - **既存コード再利用**: generic_case_a の `_with_scope` 関数を内部で呼び出し
+//!
+//! ## 使用例
+//!
+//! ```ignore
+//! let lowerer = LoopToJoinLowerer::new();
+//! let join_module = lowerer.lower(func, &loop_form, &query)?;
+//! ```
+
+use crate::mir::join_ir::lowering::generic_case_a;
+use crate::mir::join_ir::lowering::loop_form_intake::intake_loop_form;
+use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
+use crate::mir::join_ir::JoinModule;
+use crate::mir::loop_form::LoopForm;
+use crate::mir::phi_core::loop_exit_liveness::LoopExitLivenessBox;
+use crate::mir::phi_core::loop_var_classifier::LoopVarClassBox;
+use crate::mir::query::MirQueryBox;
+use crate::mir::MirFunction;
+
+/// Loop→JoinIR 変換の統一箱
+///
+/// Phase 31 で導入された統一インターフェース。
+/// 全ての MIR LoopForm を JoinIR に正規化する。
+pub struct LoopToJoinLowerer {
+    /// デバッグモード（詳細ログ出力）
+    debug: bool,
+}
+
+impl Default for LoopToJoinLowerer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl LoopToJoinLowerer {
+    /// 新しい LoopToJoinLowerer を作成
+    pub fn new() -> Self {
+        let debug = std::env::var("NYASH_LOOPTOJOIN_DEBUG")
+            .map(|v| v == "1")
+            .unwrap_or(false);
+        Self { debug }
+    }
+
+    /// MIR LoopForm を JoinIR に変換
+    ///
+    /// # Arguments
+    ///
+    /// - `func`: MIR 関数
+    /// - `loop_form`: 変換対象の LoopForm
+    /// - `func_name`: 関数名（オプション、ルーティング用）
+    ///
+    /// # Returns
+    ///
+    /// - `Some(JoinModule)`: 変換成功
+    /// - `None`: 変換失敗（フォールバック経路へ）
+    pub fn lower(
+        &self,
+        func: &MirFunction,
+        loop_form: &LoopForm,
+        func_name: Option<&str>,
+    ) -> Option<JoinModule> {
+        if self.debug {
+            eprintln!(
+                "[LoopToJoinLowerer] lower() called for {:?}",
+                func_name.unwrap_or("<unknown>")
+            );
+        }
+
+        // Step 1: MirQuery を構築
+        let query = MirQueryBox::new(func);
+
+        // Step 2: 分類箱を構築（Phase 31 では空箱、将来 MIR 解析で埋める）
+        let var_classes = LoopVarClassBox::new();
+        let exit_live = LoopExitLivenessBox::new();
+
+        // Step 3: LoopFormIntake を構築
+        let intake = intake_loop_form(loop_form, &var_classes, &query, func)?;
+
+        // Step 4: LoopScopeShape を構築
+        let scope =
+            LoopScopeShape::from_existing_boxes(loop_form, &intake, &var_classes, &exit_live, &query, func_name)?;
+
+        if self.debug {
+            eprintln!(
+                "[LoopToJoinLowerer] LoopScopeShape built: pinned={:?}, carriers={:?}, exit_live={:?}",
+                scope.pinned, scope.carriers, scope.exit_live
+            );
+        }
+
+        // Step 5: パターンに応じた lowering を実行
+        self.lower_with_scope(scope, func_name)
+    }
+
+    /// LoopScopeShape から JoinModule を生成（内部メソッド）
+    fn lower_with_scope(
+        &self,
+        scope: LoopScopeShape,
+        func_name: Option<&str>,
+    ) -> Option<JoinModule> {
+        let name = func_name.unwrap_or("");
+
+        // Case A minimal targets へのディスパッチ
+        if name.contains("skip") || name == "Main.skip/1" {
+            if self.debug {
+                eprintln!("[LoopToJoinLowerer] dispatching to skip_ws lowerer");
+            }
+            return generic_case_a::lower_case_a_skip_ws_with_scope(scope);
+        }
+
+        if name.contains("trim") || name == "FuncScannerBox.trim/1" {
+            if self.debug {
+                eprintln!("[LoopToJoinLowerer] dispatching to trim lowerer");
+            }
+            return generic_case_a::lower_case_a_trim_with_scope(scope);
+        }
+
+        if name.contains("append_defs") || name == "FuncScannerBox.append_defs/2" {
+            if self.debug {
+                eprintln!("[LoopToJoinLowerer] dispatching to append_defs lowerer");
+            }
+            return generic_case_a::lower_case_a_append_defs_with_scope(scope);
+        }
+
+        if name.contains("resolve_for_source") || name == "Stage1UsingResolverBox.resolve_for_source/5" {
+            if self.debug {
+                eprintln!("[LoopToJoinLowerer] dispatching to stage1 lowerer");
+            }
+            return generic_case_a::lower_case_a_stage1_usingresolver_with_scope(scope);
+        }
+
+        // TODO: Phase 31 Step 3 で汎用 Case A 対応
+        if self.debug {
+            eprintln!(
+                "[LoopToJoinLowerer] no matching pattern for {:?}, returning None",
+                name
+            );
+        }
+
+        None
+    }
+
+    // ========================================
+    // Convenience methods for specific patterns
+    // ========================================
+
+    /// Main.skip/1 専用の lowering
+    ///
+    /// Phase 31 Step 1 の検証用メソッド。
+    /// 既存の `skip_ws.rs` からの移行確認に使用。
+    pub fn lower_minimal_skip_ws_case_a(
+        &self,
+        func: &MirFunction,
+        loop_form: &LoopForm,
+    ) -> Option<JoinModule> {
+        self.lower(func, loop_form, Some("Main.skip/1"))
+    }
+
+    /// FuncScannerBox.trim/1 専用の lowering
+    pub fn lower_minimal_trim_case_a(
+        &self,
+        func: &MirFunction,
+        loop_form: &LoopForm,
+    ) -> Option<JoinModule> {
+        self.lower(func, loop_form, Some("FuncScannerBox.trim/1"))
+    }
+
+    /// FuncScannerBox.append_defs/2 専用の lowering
+    pub fn lower_minimal_append_defs_case_a(
+        &self,
+        func: &MirFunction,
+        loop_form: &LoopForm,
+    ) -> Option<JoinModule> {
+        self.lower(func, loop_form, Some("FuncScannerBox.append_defs/2"))
+    }
+
+    /// Stage1UsingResolverBox.resolve_for_source/5 専用の lowering
+    pub fn lower_minimal_stage1_case_a(
+        &self,
+        func: &MirFunction,
+        loop_form: &LoopForm,
+    ) -> Option<JoinModule> {
+        self.lower(func, loop_form, Some("Stage1UsingResolverBox.resolve_for_source/5"))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_lowerer_creation() {
+        let lowerer = LoopToJoinLowerer::new();
+        assert!(!lowerer.debug || lowerer.debug); // Just check it compiles
+    }
+}
diff --git a/src/mir/join_ir/lowering/mod.rs b/src/mir/join_ir/lowering/mod.rs
index d9232970..1b5f975c 100644
--- a/src/mir/join_ir/lowering/mod.rs
+++ b/src/mir/join_ir/lowering/mod.rs
@@ -20,6 +20,7 @@ pub mod funcscanner_trim;
 pub mod generic_case_a;
 pub mod loop_form_intake;
 pub mod loop_scope_shape;
+pub mod loop_to_join;
 pub mod min_loop;
 pub mod skip_ws;
 pub mod stage1_using_resolver;
@@ -30,6 +31,8 @@ pub mod value_id_ranges;
 // Re-export public lowering functions
 pub use funcscanner_append_defs::lower_funcscanner_append_defs_to_joinir;
 pub use funcscanner_trim::lower_funcscanner_trim_to_joinir;
+// Phase 31: LoopToJoinLowerer 統一箱
+pub use loop_to_join::LoopToJoinLowerer;
 // Phase 30 F-3: 旧 lower_case_a_loop_to_joinir_for_minimal_skip_ws は _with_scope に置き換え済みのため削除
 pub use min_loop::lower_min_loop_to_joinir;
 pub use skip_ws::lower_skip_ws_to_joinir;