Demo
HTML&CSS
<div class="form_simple1">
<form action="" method="post">
<p><label for="post_number">郵便番号</label></p>
<input type="text" name="post_number" id="post_number">
<p><label for="address">住所</label></p>
<input type="text" name="address" id="address">
<p><label for="name">氏名</label></p>
<input type="text" name="name" id="name">
<p><label for="tel">電話番号</label></p>
<input type="text" name="tel" id="tel">
<p><label for="e_mail">メールアドレス</label></p>
<input type="text" name="e_mail" id="e_mail">
<button type="submit" name="send" value="form_simple1">送信</button>
</form>
</div>
.form_simple1{
width: 80%;
text-align: left;
margin-left: auto;
margin-right: auto;
padding: 2rem;
border: 1px solid #ccc;
}
.form_simple1 input{
display: block;
width: 100%;
height: 2rem;
outline: none;
margin-bottom: 1rem;
border: 1px solid #ccc;
}
.form_simple1 button{
display: block;
cursor: pointer;
outline: none;
padding: 1rem 2rem;
margin-top: 2rem;
margin-left: auto;
margin-right: auto;
border: 1px solid #ccc;
}