diff --git a/views/404.php b/views/404.php
index 544b381..e48804e 100644
--- a/views/404.php
+++ b/views/404.php
@@ -1,4 +1,4 @@
 <?php include_once('includes/header.php') ?>
-      このURLを見つけられませんでした。<br />
+      <?php echo $err; ?><br />
       <a href="/">トップページに戻る</a>
 <?php include_once('includes/footer.php'); ?>
diff --git a/www/index.php b/www/index.php
index 67d13b1..4dd220d 100644
--- a/www/index.php
+++ b/www/index.php
@@ -19,9 +19,26 @@
   // パラメートルがなければ、スキップ。
   if (isset($_POST['sosin'])) {
     $newadd = $_POST['newadd'];
+    if (!str_starts_with($newadd, 'http://') && !str_starts_with($newadd, 'https://')) {
+      $title = '076エラー - 不正なURL';
+      $h1 = '【エラー】不正なURL';
+      $bg = 'daityan';
+      $bgpos = 'top right 100px';
+      $err = 'URLは「http://」又は「https://」で始まります。';
+      include_once('../views/404.php');
+      exit;
+    }
+
     $data = file_get_contents('links.json');
 
     $arr = json_decode($data, true);
+    foreach ($arr as $k => $v) {
+      if ($newadd == $v) {
+        header('Location: /?newurl='.$k);
+        exit;
+      }
+    }
+
     $newstring = mkstring();
 
     $arr[$newstring] = $newadd;
@@ -29,6 +46,7 @@
     file_put_contents('links.json', $newdata);
 
     header('Location: /?newurl='.$newstring);
+    exit;
   }
   else if (null === $_GET['url'] && null !== $_GET['newurl']) {
     $newurl = $_GET['newurl'];
@@ -57,6 +75,7 @@
     $h1 = '【４０４】未検出';
     $bg = 'daityan';
     $bgpos = 'top right 100px';
+    $err = 'このURLを見つけられませんでした。';
     include_once('../views/404.php');
   }
 ?>