.openbtn {
  display: none;
}

@media (width <=768px) {
  .header-nav {
    position: fixed;
    z-index: 999;
    top: 0;
    right: -120%;
    width: 100%;
    height: 100vh;
    background: #767676ea;
    transition: all 0.6s;
    display: block;
  }

  .header-nav.panelactive {
    right: 0;
  }

  .header-nav.panelactive #g-nav-list {
    /*ナビの数が増えた場合縦スクロール*/
    position: fixed;
    z-index: 999;
    width: 100%;
    height: 100vh;
    /*表示する高さ*/
    overflow: auto;
    -webkit-overflow-scrolling: touch;
  }

  .header-nav ul {
    position: absolute;
    z-index: 999;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
  }

  .header-nav li {
    list-style: none;
    text-align: center;
    color: white;
  }

  .openbtn {
    display: block;
    position: fixed;
    z-index: 9999;
    /*ボタンを最前面に*/
    top: 35px;
    right: 16px;
    cursor: pointer;
    width: 60px;
    height: 60px;
  }

  .openbtn span {
    display: inline-block;
    transition: all .4s;
    position: absolute;
    left: 14px;
    height: 3px;
    border-radius: 2px;
    background-color: #000000;
    width: 45%;
  }

  .openbtn span:nth-of-type(1) {
    top: 15px;
  }

  .openbtn span:nth-of-type(2) {
    top: 23px;
  }

  .openbtn span:nth-of-type(3) {
    top: 31px;
  }

  .openbtn.active span:nth-of-type(1) {
    top: 18px;
    left: 18px;
    transform: translateY(6px) rotate(-45deg);
    width: 30%;
    background-color: white;
  }

  .openbtn.active span:nth-of-type(2) {
    opacity: 0;
  }

  .openbtn.active span:nth-of-type(3) {
    top: 30px;
    left: 18px;
    transform: translateY(-6px) rotate(45deg);
    width: 30%;
    background-color: white;
  }
}