diff --git a/laravel-app/app/Http/Controllers/BookController.php b/laravel-app/app/Http/Controllers/BookController.php
index 093d108..bf0de31 100644
--- a/laravel-app/app/Http/Controllers/BookController.php
+++ b/laravel-app/app/Http/Controllers/BookController.php
@@ -54,31 +54,35 @@ public function store(Request $request)
             'thumbnail_url' => 'nullable|url',
         ]);
 
-        // 同じISBNの最大copy_numberを取得して+1
-        $maxCopy = Book::where('isbn', $request->isbn)->max('copy_number') ?? 0;
-        $copyNumber = $maxCopy + 1;
-
-        // データベースに保存
-        $book = Book::create([
-            'title' => $request->title,
-            'author' => $request->author,
-            'isbn' => $request->isbn,
-            'copy_number' => $copyNumber,
-            'publisher' => $request->publisher,
-            'published_date' => $request->published_date,
-            'description' => $request->description,
-            'thumbnail_url' => $request->thumbnail_url,
-        ]);
+        return \Illuminate\Support\Facades\DB::transaction(function () use ($request) {
+            // 同じISBNの最大copy_numberを取得して+1（排他ロック）
+            $maxCopy = Book::where('isbn', $request->isbn)
+                ->lockForUpdate()
+                ->max('copy_number') ?? 0;
+            $copyNumber = $maxCopy + 1;
+
+            // データベースに保存
+            $book = Book::create([
+                'title' => $request->title,
+                'author' => $request->author,
+                'isbn' => $request->isbn,
+                'copy_number' => $copyNumber,
+                'publisher' => $request->publisher,
+                'published_date' => $request->published_date,
+                'description' => $request->description,
+                'thumbnail_url' => $request->thumbnail_url,
+            ]);
 
-        $copyLabel = $copyNumber > 1 ? "（冊{$copyNumber}）" : '';
-        Log::info("Book registered: {$book->title}{$copyLabel} (ID: {$book->id})");
-        AuditLog::log('book_created', $book, "{$book->title}{$copyLabel}");
+            $copyLabel = $copyNumber > 1 ? "（冊{$copyNumber}）" : '';
+            Log::info("Book registered: {$book->title}{$copyLabel} (ID: {$book->id})");
+            AuditLog::log('book_created', $book, "{$book->title}{$copyLabel}");
 
-        $message = $copyNumber > 1
-            ? "書籍を登録しました（{$copyNumber}冊目）"
-            : '書籍を登録しました';
+            $message = $copyNumber > 1
+                ? "書籍を登録しました（{$copyNumber}冊目）"
+                : '書籍を登録しました';
 
-        return redirect()->route('books.index')->with('success', $message);
+            return redirect()->route('books.index')->with('success', $message);
+        });
     } 
 
     // ISBN検索（拡張版）
diff --git a/laravel-app/app/Http/Controllers/LoanController.php b/laravel-app/app/Http/Controllers/LoanController.php
index 6138071..ccd9105 100644
--- a/laravel-app/app/Http/Controllers/LoanController.php
+++ b/laravel-app/app/Http/Controllers/LoanController.php
@@ -75,6 +75,11 @@ public function returnBook(Loan $loan)
             ->with('error', '他人の貸出記録は操作できません');
         }
 
+        // 既に返却済みかチェック
+        if ($loan->returned_at) {
+            return redirect()->route('loans.my')->with('error', 'この貸出は既に返却済みです');
+        }
+
         // 返却処理
         $loan->update([
             'returned_at' => now(),
diff --git a/laravel-app/app/Http/Controllers/ProfileController.php b/laravel-app/app/Http/Controllers/ProfileController.php
index a48eb8d..bf8eaca 100644
--- a/laravel-app/app/Http/Controllers/ProfileController.php
+++ b/laravel-app/app/Http/Controllers/ProfileController.php
@@ -46,6 +46,13 @@ public function destroy(Request $request): RedirectResponse
             'password' => ['required', 'current_password'],
         ]);
 
+        // 貸出中の本があれば削除を拒否
+        $activeLoanCount = $request->user()->loans()
+            ->whereNull('returned_at')->count();
+        if ($activeLoanCount > 0) {
+            return back()->with('error', '貸出中の本があるため、アカウントを削除できません。先に返却してください。');
+        }
+
         $user = $request->user();
 
         Auth::logout();
diff --git a/laravel-app/app/Http/Middleware/SecurityHeadersMiddleware.php b/laravel-app/app/Http/Middleware/SecurityHeadersMiddleware.php
index 3521e37..7966a83 100644
--- a/laravel-app/app/Http/Middleware/SecurityHeadersMiddleware.php
+++ b/laravel-app/app/Http/Middleware/SecurityHeadersMiddleware.php
@@ -18,6 +18,10 @@ public function handle(Request $request, Closure $next): Response
         $response->headers->set('Referrer-Policy', 'no-referrer-when-downgrade');
         $response->headers->set('Content-Security-Policy', "default-src 'self' http: https: data: blob: 'unsafe-inline' 'unsafe-eval'");
 
+        if (app()->environment('production')) {
+            $response->headers->set('Strict-Transport-Security', 'max-age=31536000; includeSubDomains');
+        }
+
         return $response;
     }
 }
diff --git a/laravel-app/routes/api.php b/laravel-app/routes/api.php
index 4dcf550..d94923d 100644
--- a/laravel-app/routes/api.php
+++ b/laravel-app/routes/api.php
@@ -19,8 +19,8 @@
     return $request->user();
 });
 
-// ISBN検索API
-Route::get('/book/info/{isbn}', function (Request $request, $isbn) {
+// ISBN検索API（1分あたり30回まで）
+Route::middleware('throttle:30,1')->get('/book/info/{isbn}', function (Request $request, $isbn) {
     try {
         $service = app(BookSearchService::class);
         $bookData = $service->fetchByIsbn($isbn);