diff --git a/server/src/converters/convert_image.js b/server/src/converters/convert_image.js
index 4ad26c2ae..c9c391c92 100644
--- a/server/src/converters/convert_image.js
+++ b/server/src/converters/convert_image.js
@@ -9,6 +9,16 @@ import sharp from 'sharp';
  * @returns {Promise<Uint8Array>}
  */
 async function convertImage(buffer, options) {
+  if (options.extension === 'webp') {
+    return sharp(buffer)
+      .resize({
+        fit: 'cover',
+        height: options.height,
+        width: options.width,
+      })
+      .webp({ lossless: true })
+      .toBuffer();
+  }
   return sharp(buffer)
     .resize({
       fit: 'cover',
diff --git a/server/src/routes/api/image.js b/server/src/routes/api/image.js
index 214d5980e..1690fee4b 100644
--- a/server/src/routes/api/image.js
+++ b/server/src/routes/api/image.js
@@ -9,7 +9,7 @@ import { convertImage } from '../../converters/convert_image';
 import { UPLOAD_PATH } from '../../paths';
 
 // 変換した画像の拡張子
-const EXTENSION = 'jpg';
+const EXTENSION = 'webp';
 
 const router = Router();
 
@@ -29,7 +29,7 @@ router.post('/images', async (req, res) => {
     // 画像の縦サイズを指定する (undefined は元画像に合わせる)
     height: undefined,
     // 画像の横サイズを指定する (undefined は元画像に合わせる)
-    width: undefined,
+    width: 600,
   });
 
   const filePath = path.resolve(UPLOAD_PATH, `./images/${imageId}.${EXTENSION}`);