diff --git a/src/mir/builder.rs b/src/mir/builder.rs
index 25e309fc..cbe413e3 100644
--- a/src/mir/builder.rs
+++ b/src/mir/builder.rs
@@ -125,35 +125,40 @@ impl MirBuilder {
     
     /// Build a complete MIR module from AST
     pub fn build_module(&mut self, ast: ASTNode) -> Result<MirModule, String> {
-        // Create a new module
+        self.prepare_module()?;
+        let result_value = self.lower_root(ast)?;
+        self.finalize_module(result_value)
+    }
+
+    fn prepare_module(&mut self) -> Result<(), String> {
         let module = MirModule::new("main".to_string());
-        
-        // Create a main function to contain the AST
         let main_signature = FunctionSignature {
             name: "main".to_string(),
             params: vec![],
             return_type: MirType::Void,
             effects: EffectMask::PURE,
         };
-        
+
         let entry_block = self.block_gen.next();
         let mut main_function = MirFunction::new(main_signature, entry_block);
         main_function.metadata.is_entry_point = true;
-        
-        // Set up building context
+
         self.current_module = Some(module);
         self.current_function = Some(main_function);
         self.current_block = Some(entry_block);
-        
-        // Optional: Add safepoint at function entry (disabled by default)
+
         if std::env::var("NYASH_BUILDER_SAFEPOINT_ENTRY").ok().as_deref() == Some("1") {
             self.emit_instruction(MirInstruction::Safepoint)?;
         }
-        
-        // Convert AST to MIR
-        let result_value = self.build_expression(ast)?;
-        
-        // Add return instruction if needed
+
+        Ok(())
+    }
+
+    fn lower_root(&mut self, ast: ASTNode) -> Result<ValueId, String> {
+        self.build_expression(ast)
+    }
+
+    fn finalize_module(&mut self, result_value: ValueId) -> Result<MirModule, String> {
         if let Some(block_id) = self.current_block {
             if let Some(ref mut function) = self.current_function {
                 if let Some(block) = function.get_block_mut(block_id) {
@@ -162,41 +167,48 @@ impl MirBuilder {
                             value: Some(result_value),
                         });
                     }
-                    // Infer return type from TyEnv (value_types)
                     if let Some(mt) = self.value_types.get(&result_value).cloned() {
                         function.signature.return_type = mt;
                     }
                 }
             }
         }
-        
-        // Finalize and return module
+
         let mut module = self.current_module.take().unwrap();
         let mut function = self.current_function.take().unwrap();
-        // Flush value_types (TyEnv) into function metadata
         function.metadata.value_types = self.value_types.clone();
-        // 補助: 本文中に明示的な return <expr> が存在する場合、
-        // 末尾returnを挿入しなかった関数でも戻り型を推定する。
         if matches!(function.signature.return_type, super::MirType::Void | super::MirType::Unknown) {
             let mut inferred: Option<super::MirType> = None;
             'outer: for (_bid, bb) in function.blocks.iter() {
                 for inst in bb.instructions.iter() {
                     if let super::MirInstruction::Return { value: Some(v) } = inst {
-                        if let Some(mt) = self.value_types.get(v).cloned() { inferred = Some(mt); break 'outer; }
-                        // 追加: v が PHI の場合は入力側の型から推定
-                if let Some(mt) = phi::infer_type_from_phi(&function, *v, &self.value_types) { inferred = Some(mt); break 'outer; }
+                        if let Some(mt) = self.value_types.get(v).cloned() {
+                            inferred = Some(mt);
+                            break 'outer;
+                        }
+                        if let Some(mt) = phi::infer_type_from_phi(&function, *v, &self.value_types) {
+                            inferred = Some(mt);
+                            break 'outer;
+                        }
                     }
                 }
                 if let Some(super::MirInstruction::Return { value: Some(v) }) = &bb.terminator {
-                    if let Some(mt) = self.value_types.get(v).cloned() { inferred = Some(mt); break; }
-                    if let Some(mt) = phi::infer_type_from_phi(&function, *v, &self.value_types) { inferred = Some(mt); break; }
+                    if let Some(mt) = self.value_types.get(v).cloned() {
+                        inferred = Some(mt);
+                        break;
+                    }
+                    if let Some(mt) = phi::infer_type_from_phi(&function, *v, &self.value_types) {
+                        inferred = Some(mt);
+                        break;
+                    }
                 }
             }
-            if let Some(mt) = inferred { function.signature.return_type = mt; }
+            if let Some(mt) = inferred {
+                function.signature.return_type = mt;
+            }
         }
         module.add_function(function);
 
-        
         Ok(module)
     }