diff --git a/backend/app/main.py b/backend/app/main.py
index afe990b..3985786 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -2,7 +2,7 @@ from dotenv import load_dotenv
 load_dotenv()
 
 from fastapi import FastAPI, Request
-from fastapi.responses import JSONResponse
+from fastapi.responses import JSONResponse, RedirectResponse
 from pydantic import ValidationError
 from fastapi.middleware.cors import CORSMiddleware
 
@@ -32,9 +32,9 @@ async def validation_exception_handler(request: Request, exc: ValidationError):
 # ─────────────────────────
 # ルート（稼働確認）
 # ─────────────────────────
-@app.get("/", include_in_schema=False)
-def root():
-    return RedirectResponse(url="/index.html")
+#@app.get("/", include_in_schema=False)
+#def root():
+#  return RedirectResponse(url="/index.html")
 
 # ─────────────────────────
 # ルーター登録（※ 必ず app 定義の後）
@@ -124,10 +124,18 @@ app.mount(
 # )
 
 # フロントエンドファイル配信
-frontend_path = os.path.join(os.path.dirname(os.path.dirname(os.path.dirname(__file__))), "frontend")
-if os.path.exists(frontend_path):
+from fastapi.staticfiles import StaticFiles
+from pathlib import Path
+
+# フロントエンド配信（/app/frontend）
+BASE_DIR = Path(__file__).resolve().parent.parent   # /app
+frontend_path = BASE_DIR / "frontend"
+
+print("Frontend path =", frontend_path)
+
+if frontend_path.exists():
     app.mount(
-        "/frontend",
+        "/",
         StaticFiles(directory=frontend_path, html=True),
         name="frontend",
     )
diff --git a/backend/app/payroll/settings/router.py b/backend/app/payroll/settings/router.py
index e223395..51819a7 100644
--- a/backend/app/payroll/settings/router.py
+++ b/backend/app/payroll/settings/router.py
@@ -204,6 +204,15 @@ def create_insurance_rate(rate: schemas.InsuranceRateCreate):
     """保険料率を作成"""
     with get_connection() as conn:
         with conn.cursor() as cur:
+            # 既存の同キー（rate_year, rate_type, calculation_target, prefecture）が存在しないか確認
+            cur.execute(
+                "SELECT 1 FROM insurance_rates WHERE rate_year = %s AND rate_type = %s AND calculation_target = %s AND prefecture = %s LIMIT 1",
+                (rate.rate_year, rate.rate_type, rate.calculation_target, rate.prefecture)
+            )
+            if cur.fetchone():
+                # 重複があればクライアントに分かりやすく 409 を返す
+                raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail="指定された年度・種類・計算対象・都道府県の保険料率は既に存在します")
+
             cur.execute(
                 """
                 INSERT INTO insurance_rates (
diff --git a/docker-compose.yml b/docker-compose.yml
index 969057c..bc168fc 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -18,7 +18,7 @@ services:
     container_name: njts_nginx
     restart: unless-stopped
     ports:
-      - "80:80"
+      - "18000:80"
     volumes:
       - ./frontend:/usr/share/nginx/html:ro
       - ./docker/nginx/default.conf:/etc/nginx/conf.d/default.conf:ro
diff --git a/docker/nginx/default.conf b/docker/nginx/default.conf
index 9089eb7..ed5fbb3 100644
--- a/docker/nginx/default.conf
+++ b/docker/nginx/default.conf
@@ -7,7 +7,9 @@ server {
 
     # Serve static files
     location / {
-        try_files $uri $uri/ =404;
+        # Try to serve the exact file, then directory, otherwise fall back to index.html
+        # This ensures accessing / or any SPA route returns /index.html
+        try_files $uri $uri/ /index.html;
     }
 
     # Proxy API requests to backend container