Edit in JSFiddle

$("input").each(function(){
    $(this).attr("placeholder",
          $('label[for='+ $(this).attr('id') +']').text());
});

form{margin:25px}
label{display:none;}
div{margin-top:15px;}

<form action="" id="form" method="post">
        <div>
                <label for="name1">お名前</label>
        </div>
        <div>
                <input type="text" id="name1" name="name" value="" size="40">
        </div>
        <div>
                <label for="email1">メールアドレス</label>
        </div>
        <div>
                <input type="email" id="email1" name="email" value="" size="40">
        </div>

        <div>
                <label for="phone">電話番号</label>
        </div>
        <div>
                <input type="text" id="phone" name="phone" value="" size="40">
        </div>
<div>
  <input type="submit" id="submit" value="Submit"> 
</div>

</form>