diff --git a/src/app/api/expansions/[id]/run/route.ts b/src/app/api/expansions/[id]/run/route.ts
index a738819..ec68f35 100644
--- a/src/app/api/expansions/[id]/run/route.ts
+++ b/src/app/api/expansions/[id]/run/route.ts
@@ -12,6 +12,7 @@ import { RunExpansionSchema, DirectionSchema } from "@/lib/validation";
 import { getImageGenProvider } from "@/lib/image-gen";
 import { getUserIdFromSession } from "@/lib/auth";
 import { emitRoomEvent } from "@/lib/sse-emitter";
+import { logger } from "@/lib/logger";
 import type { Direction } from "@/types";
 
 export async function POST(
@@ -123,7 +124,7 @@ export async function POST(
       }),
     ]);
     emitRoomEvent(expansion.roomId, "room_update");
-    console.error("Image generation failed:", err instanceof Error ? err.message : String(err));
+    logger.error("Image generation failed:", err);
     return serverError("Image generation failed");
   }
 }
diff --git a/src/app/api/rooms/[id]/generate-initial/route.ts b/src/app/api/rooms/[id]/generate-initial/route.ts
index 70a1389..99d97e7 100644
--- a/src/app/api/rooms/[id]/generate-initial/route.ts
+++ b/src/app/api/rooms/[id]/generate-initial/route.ts
@@ -4,6 +4,7 @@ import { notFound, unauthorized, forbidden, conflict } from "@/lib/errors";
 import { getUserIdFromSession } from "@/lib/auth";
 import { getImageGenProvider } from "@/lib/image-gen";
 import { emitRoomEvent } from "@/lib/sse-emitter";
+import { logger } from "@/lib/logger";
 import type { PromptJson } from "@/types";
 
 /** GENERATING 状態のまま放置を許容する最大時間（5分） */
@@ -80,7 +81,7 @@ export async function POST(
 
     return NextResponse.json({ status: "DONE", imageUrl: result.imagePath });
   } catch (error) {
-    console.error("Initial tile generation failed:", error instanceof Error ? error.message : String(error));
+    logger.error("Initial tile generation failed:", error);
 
     // 失敗: status FAILED
     await prisma.room.update({
diff --git a/src/hooks/use-room.ts b/src/hooks/use-room.ts
index 811490a..5d167a0 100644
--- a/src/hooks/use-room.ts
+++ b/src/hooks/use-room.ts
@@ -1,4 +1,5 @@
 import { useState, useEffect, useCallback, useRef } from "react";
+import { logger } from "@/lib/logger";
 import type { RoomDetail } from "@/types";
 
 const FALLBACK_POLL_INTERVAL = 3_000;
@@ -21,7 +22,7 @@ export function useRoom(roomId: string) {
         setError(errData.message || "Failed to fetch room");
       }
     } catch (err) {
-      console.error("Failed to fetch room:", err instanceof Error ? err.message : String(err));
+      logger.error("Failed to fetch room:", err);
       setError("Network error");
     } finally {
       setLoading(false);
@@ -56,7 +57,7 @@ export function useRoom(roomId: string) {
 
     eventSource.onerror = () => {
       if (!pollTimer) {
-        console.warn("SSE connection lost, falling back to polling");
+        logger.warn("SSE connection lost, falling back to polling");
         pollTimer = setInterval(fetchRoom, FALLBACK_POLL_INTERVAL);
       }
     };
diff --git a/src/hooks/use-user.ts b/src/hooks/use-user.ts
index 79986ea..abc4b7c 100644
--- a/src/hooks/use-user.ts
+++ b/src/hooks/use-user.ts
@@ -1,4 +1,5 @@
 import { useState, useEffect, useCallback } from "react";
+import { logger } from "@/lib/logger";
 
 export interface User {
   id: string;
@@ -19,7 +20,7 @@ export function useUser() {
         setUser(null);
       }
     } catch (error) {
-      console.error("Failed to fetch user:", error instanceof Error ? error.message : String(error));
+      logger.error("Failed to fetch user:", error);
       setUser(null);
     } finally {
       setLoading(false);
diff --git a/src/lib/auto-adopt.ts b/src/lib/auto-adopt.ts
index c0ac2c2..e97db5e 100644
--- a/src/lib/auto-adopt.ts
+++ b/src/lib/auto-adopt.ts
@@ -2,6 +2,7 @@ import { createId } from "@paralleldrive/cuid2";
 import { Prisma } from "@prisma/client";
 import { prisma } from "./prisma";
 import { emitRoomEvent } from "./sse-emitter";
+import { logger } from "./logger";
 
 // DONE 状態のまま放置された Expansion を自動決定するまでの時間 (ms)
 // デフォルト 1 分。AUTO_ADOPT_AFTER_MS 環境変数で変更可能。
@@ -115,9 +116,9 @@ export async function autoAdoptStaleExpansions(roomId: string): Promise<void> {
   } catch (err) {
     // バッチ処理が失敗した場合（例: ユニーク制約違反）、
     // 堅牢性のために従来の逐次処理にフォールバックする
-    console.warn(
+    logger.warn(
       `[auto-adopt] Batched transaction failed, falling back to sequential:`,
-      err instanceof Error ? err.message : String(err)
+      err
     );
     await autoAdoptSequentialFallback(byCell, roomId);
   }
@@ -218,22 +219,14 @@ async function autoAdoptSequentialFallback(
         ]);
         changed = true;
       } catch (err) {
-        console.warn(
-          `[auto-adopt] expansion ${pick.id} fallback failed:`,
-          err instanceof Error ? err.message : String(err)
-        );
+        logger.warn(`[auto-adopt] expansion ${pick.id} fallback failed:`, err);
         // P2: リトライループ防止 — 失敗した全候補を REJECTED に更新
         await prisma.expansion
           .updateMany({
             where: { id: { in: candidates.map((e) => e.id) }, status: "DONE" },
             data: { status: "REJECTED" },
           })
-          .catch((e2) =>
-            console.warn(
-              `[auto-adopt] fallback reject failed:`,
-              e2 instanceof Error ? e2.message : String(e2)
-            )
-          );
+          .catch((e2) => logger.warn(`[auto-adopt] fallback reject failed:`, e2));
       }
     } else {
       // 全却下
@@ -253,10 +246,7 @@ async function autoAdoptSequentialFallback(
         ]);
         changed = true;
       } catch (err) {
-        console.warn(
-          `[auto-adopt] all-reject for cell failed:`,
-          err instanceof Error ? err.message : String(err)
-        );
+        logger.warn(`[auto-adopt] all-reject for cell failed:`, err);
       }
     }
   }
diff --git a/src/lib/logger.ts b/src/lib/logger.ts
new file mode 100644
index 0000000..2aa86ac
--- /dev/null
+++ b/src/lib/logger.ts
@@ -0,0 +1,24 @@
+/**
+ * Centralized logger.
+ * Error objects output stack trace (or message if unavailable) for debuggability,
+ * while preventing raw Error objects from being passed to console methods.
+ */
+
+const formatArg = (arg: unknown): unknown => {
+  if (arg instanceof Error) {
+    return arg.stack || arg.message;
+  }
+  return arg;
+};
+
+export const logger = {
+  info: (...args: unknown[]) => {
+    console.info(...args.map(formatArg));
+  },
+  warn: (...args: unknown[]) => {
+    console.warn(...args.map(formatArg));
+  },
+  error: (...args: unknown[]) => {
+    console.error(...args.map(formatArg));
+  },
+};