diff --git a/src/api/endpoints/posts/create.js b/src/api/endpoints/posts/create.js
index cdcbf4f966..694e6fc824 100644
--- a/src/api/endpoints/posts/create.js
+++ b/src/api/endpoints/posts/create.js
@@ -17,7 +17,7 @@ import event from '../../event';
 /**
  * 最大文字数
  */
-const maxTextLength = 300;
+const maxTextLength = 500;
 
 /**
  * 添付できるファイルの数
diff --git a/src/web/app/desktop/tags/post-form.tag b/src/web/app/desktop/tags/post-form.tag
index 9683859477..8ddc9e0b00 100644
--- a/src/web/app/desktop/tags/post-form.tag
+++ b/src/web/app/desktop/tags/post-form.tag
@@ -12,7 +12,7 @@
 	<mk-uploader ref="uploader"></mk-uploader>
 	<button ref="upload" title="PCからファイルを添付" onclick={ selectFile }><i class="fa fa-upload"></i></button>
 	<button ref="drive" title="ドライブからファイルを添付" onclick={ selectFileFromDrive }><i class="fa fa-cloud"></i></button>
-	<p class="text-count { over: refs.text.value.length &gt; 300 }">のこり{ 300 - refs.text.value.length }文字</p>
+	<p class="text-count { over: refs.text.value.length &gt; 500 }">のこり{ 500 - refs.text.value.length }文字</p>
 	<button class={ wait: wait } ref="submit" disabled={ wait || (refs.text.value.length == 0 && files.length == 0) } onclick={ post }>{ wait ? '投稿中' : opts.reply ? '返信' : '投稿' }
 		<mk-ellipsis if={ wait }></mk-ellipsis>
 	</button>
diff --git a/src/web/app/mobile/tags/post-form.tag b/src/web/app/mobile/tags/post-form.tag
index e2d0c96024..28e508ebc5 100644
--- a/src/web/app/mobile/tags/post-form.tag
+++ b/src/web/app/mobile/tags/post-form.tag
@@ -2,7 +2,7 @@
 	<header>
 		<div>
 			<button class="cancel" onclick={ cancel }><i class="fa fa-times"></i></button>
-			<div><span class="text-count { over: refs.text.value.length &gt; 300 }">{ 300 - refs.text.value.length }</span>
+			<div><span class="text-count { over: refs.text.value.length &gt; 500 }">{ 500 - refs.text.value.length }</span>
 				<button class="submit" onclick={ post }>投稿</button>
 			</div>
 		</div>