@media (max-width: 500px) {
  * {
    margin: 0;
    padding: 0;
    font-family: "MS Mincho";
    list-style: none;
    text-decoration: none;
  }

  :root {
    --section-border: 700px;
  }

  header {
    display: flex;
    justify-content: space-between;
    width: 100%;
    height: 100px;
    margin: 0 auto;
    /* position: static; */
    /* background-color: #c9c9c9; */
    /* opacity: 0.7; */
    /* position: sticky; */
    /* top: 0; */
  }

  .top_text {
    display: flex;
    margin: auto auto auto 0;
  }

  h1 {
    line-height: 100px;
    text-align: left;
    margin: 0 0 0 20%;
  }

  .header_nav {
    margin: auto 0 auto auto;
    width: 50%;
  }

  .header_ul {
    width: 100%;
    display: flex;
    margin: 0 0 0 25%;
    /* justify-content: space-around; */
  }

  .header_ul a {
    font-size: 1.2em;
    color: #000;
    margin: 0 16px 0 0;
  }
}