diff --git a/backend/src/main/java/com/example/todoapp/service/StocksService.java b/backend/src/main/java/com/example/todoapp/service/StocksService.java
index bd2dc19..51a142f 100644
--- a/backend/src/main/java/com/example/todoapp/service/StocksService.java
+++ b/backend/src/main/java/com/example/todoapp/service/StocksService.java
@@ -1,6 +1,5 @@
 package com.example.todoapp.service;
 
-import com.example.todoapp.dto.StockDTO;
 import com.example.todoapp.dto.AddStocksDTO;
 import com.example.todoapp.dto.UpdateStockRequest;
 import com.example.todoapp.model.Stocks;
@@ -57,9 +56,6 @@ public class StocksService {
         } else {
             // 材料情報を取得
             Optional<Stuffs> existstuffs = stuffsRepository.findById(stock.getStuffId());
-            if (existstuffs == null) {
-                throw new RuntimeException("材料がありません");
-            }
             stuffs = existstuffs.get();
         }
 
diff --git a/backend/src/main/java/com/example/todoapp/service/ToBuysService.java b/backend/src/main/java/com/example/todoapp/service/ToBuysService.java
index 678aae6..4de730a 100644
--- a/backend/src/main/java/com/example/todoapp/service/ToBuysService.java
+++ b/backend/src/main/java/com/example/todoapp/service/ToBuysService.java
@@ -91,14 +91,22 @@ public class ToBuysService {
             stuff = optStuff.get();
         }
 
-        ToBuys toBuys = new ToBuys();
-        toBuys.setUser(user);
-        toBuys.setStuff(stuff);
-        toBuys.setAmount(toBuyDTO.getAmount());
-        toBuys.setStore(toBuyDTO.getShop());
+        Optional<ToBuys> existingToBuy = toBuysRepository.findByUserAndStuff(user, stuff);
 
-        // データベースに保存
-        return toBuysRepository.save(toBuys);
+        if (existingToBuy.isPresent()) {
+            // 存在する場合は数量を更新
+            ToBuys existing = existingToBuy.get();
+            existing.setAmount(existing.getAmount() + toBuyDTO.getAmount());
+            return toBuysRepository.save(existing);
+        } else {
+            // 新しい材料を作成
+            ToBuys toBuys = new ToBuys();
+            toBuys.setUser(user);
+            toBuys.setStuff(stuff);
+            toBuys.setAmount(toBuyDTO.getAmount());
+            toBuys.setStore(toBuyDTO.getShop());
+            return toBuysRepository.save(toBuys);
+        }
 
     }
 
diff --git a/frontend/src/pages/StockPage.tsx b/frontend/src/pages/StockPage.tsx
index 3379f18..2aa95ba 100644
--- a/frontend/src/pages/StockPage.tsx
+++ b/frontend/src/pages/StockPage.tsx
@@ -30,6 +30,14 @@ import { STOCK_ERRORS } from '../constants/errorMessages';
 import DatePicker, { registerLocale } from 'react-datepicker';
 import { ja } from 'date-fns/locale/ja'; // date-fnsの日本語ロケールをインポート
 
+// 日付をyyyy-MM-dd形式で返す関数
+const formatDateLocal = (date: Date) => {
+  const year = date.getFullYear();
+  const month = (date.getMonth() + 1).toString().padStart(2, '0');
+  const day = date.getDate().toString().padStart(2, '0');
+  return `${year}-${month}-${day}`;
+};
+
 // 新規在庫の初期状態
 const EMPTY_STOCK: Omit<Stock, 'stockId' | 'stuffId'> & { stuffId: number | null } & { newAddition: boolean } = {
   stuffId: null,
@@ -494,7 +502,7 @@ const StockPage: React.FC = () => {
                   popperClassName="custom-datepicker-popper"
                   selected={newStock.buyDate ? new Date(newStock.buyDate) : null}
                   onChange={(date) =>
-                    setNewStock({ ...newStock, buyDate: date ? date.toISOString().split('T')[0] : '' })
+                    setNewStock({ ...newStock, buyDate: date ? formatDateLocal(date) : '' })
                   }
                   dateFormat="yyyy/MM/dd"
                   customInput={
@@ -521,7 +529,7 @@ const StockPage: React.FC = () => {
                   popperClassName="custom-datepicker-popper"
                   selected={newStock.expDate ? new Date(newStock.expDate) : null}
                   onChange={(date) =>
-                    setNewStock({ ...newStock, expDate: date ? date.toISOString().split('T')[0] : '' })
+                    setNewStock({ ...newStock, expDate: date ? formatDateLocal(date) : '' })
                   }
                   dateFormat="yyyy/MM/dd"
                   customInput={