<style>
  body {
    background-image: url("https://s3-ap-northeast-1.amazonaws.com/s3.peraichi.com/userData/21fd0434-6b4f-4bf1-8421-a1c5e73991f6/img/931cef94-04c9-4ee8-9bc9-7404aacc4c97/____________.jpg");
    background-size: cover;
    background-position: center center;
    background-repeat: no-repeat;
    background-attachment: fixed;
  }

  /* スマホでは固定背景が不安定な場合があるため通常表示に */
  @media screen and (max-width: 768px) {
    body {
      background-attachment: scroll;
    }
  }
</style>