「accordion__content」というクラス名の中に隠す内容を入れる。
「label」のfor名と「input」のid名は項目ごとにかぶらない名前を付ける。

<div class="accordionBox--type04">
  <div class="accordion">
    <input type="checkbox" id="accordion--type04__label001" />
    <label for="accordion--type04__label001">
    <div class="accordion__button">
      <p>
      <span>内容を表示する</span>
      <span>内容を閉じる</span>
      </p>
      <div class="accordion__icon">
      <div></div>
      <div></div>
      <!-- /.accordion__icon --></div>
    <!-- /.accordion__button --></div>
    </label>
    <div class="accordion__content">
      項目タイトルの閉じられている内容です。
    <!-- /.accordion__content --></div>
  <!-- /.accordion --></div>
<!--//.accordionBox--></div>
                

.accordionBox--type04 {
  overflow: hidden;
}
.accordionBox--type04 .accordion {
  position: relative;
  display: block;
  padding-bottom: 3em;
  text-align: center;
}
.accordionBox--type04 .accordion > label {
  width: 100%;
  display: block;
  position: absolute;
  bottom: 0;
  left: 0;
}
.accordionBox--type04 .accordion > label > .accordion__button {
  display: inline-block;
  padding: 0.5em 3em 0.5em 1.5em;
  background: #000;
  cursor: pointer;
  -webkit-transition: 0.3s all ease;
  transition: 0.3s all ease;
  border-radius: 3.7em;
  position: relative;
}
.accordionBox--type04 .accordion > label > .accordion__button > p {
  margin: 0;
  position: relative;
}
.accordionBox--type04 .accordion > label > .accordion__button > p > span {
  display: block;
  color: #fff;
  text-align: center;
  position: relative;
}
.accordionBox--type04 .accordion > label > .accordion__button > p > span:nth-child(2) {
  width: 100%;
  position: absolute;
  top: 0;
  left: 0;
  opacity: 0;
  -webkit-transition: 0.3s all ease;
  transition: 0.3s all ease;
}
.accordionBox--type04 .accordion > label > .accordion__button .accordion__icon {
  width: 1em;
  height: 2px;
  display: block;
  position: absolute;
  right: 1em;
  top: 50%;
  margin-top: -1px;
}
.accordionBox--type04 .accordion > label > .accordion__button .accordion__icon > div {
  background: #fff;
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  -webkit-transition: 0.4s ease;
  transition: 0.4s ease;
}
.accordionBox--type04 .accordion > label > .accordion__button .accordion__icon > div:last-child {
  -webkit-transform: rotate(90deg);
          transform: rotate(90deg);
}
.accordionBox--type04 .accordion > label > .accordion__button:hover {
  background: #666;
}
.accordionBox--type04 .accordion > input[type=checkbox] {
  display: none;
}
.accordionBox--type04 .accordion > input[type=checkbox]:checked + label + .accordion__content {
  height: auto;
  -webkit-mask-image: -webkit-gradient(linear, left bottom, left top, color-stop(10%, rgb(0, 0, 0)), to(rgb(0, 0, 0)));
  -webkit-mask-image: -webkit-linear-gradient(bottom, rgb(0, 0, 0) 10%, rgb(0, 0, 0));
          mask-image: -webkit-gradient(linear, left bottom, left top, color-stop(10%, rgb(0, 0, 0)), to(rgb(0, 0, 0)));
          mask-image: linear-gradient(0deg, rgb(0, 0, 0) 10%, rgb(0, 0, 0));
}
.accordionBox--type04 .accordion > input[type=checkbox]:checked + label > .accordion__button {
  background: #999;
  color: #fff;
}
.accordionBox--type04 .accordion > input[type=checkbox]:checked + label > .accordion__button > p > span:nth-child(1) {
  opacity: 0;
}
.accordionBox--type04 .accordion > input[type=checkbox]:checked + label > .accordion__button > p > span:nth-child(2) {
  opacity: 1;
}
.accordionBox--type04 .accordion > input[type=checkbox]:checked + label > .accordion__button:hover {
  background: #666;
  color: #000;
}
.accordionBox--type04 .accordion > input[type=checkbox]:checked + label .accordion__icon div:last-child {
  -webkit-transform: none;
          transform: none;
}
.accordionBox--type04 .accordion > .accordion__content {
  text-align: left;
  height: 8em;
  padding: 0.5em 1em;
  overflow: hidden;
  -webkit-transition: 0.5s;
  transition: 0.5s;
  display: block;
  -webkit-mask-image: -webkit-gradient(linear, left bottom, left top, color-stop(10%, rgba(0, 0, 0, 0)), to(rgb(0, 0, 0)));
  -webkit-mask-image: -webkit-linear-gradient(bottom, rgba(0, 0, 0, 0) 10%, rgb(0, 0, 0));
          mask-image: -webkit-gradient(linear, left bottom, left top, color-stop(10%, rgba(0, 0, 0, 0)), to(rgb(0, 0, 0)));
          mask-image: linear-gradient(0deg, rgba(0, 0, 0, 0) 10%, rgb(0, 0, 0));
}
              

.accordionBox--type04 {
  overflow: hidden;
  .accordion {
    position: relative;
    display: block;
    padding-bottom: 3em;
    text-align: center;
    &> label {
      width: 100%;
      display: block;
      position: absolute;
      bottom: 0;
      left: 0;
      &> .accordion__button {
        display: inline-block;
        padding: 0.5em 3em 0.5em 1.5em;
        background: #000;
        cursor: pointer;
        transition: 0.3s all ease;
        border-radius: 3.7em;
        position: relative;
        &> p {
          margin: 0;
          position: relative;
          &> span {
            display: block;
            color: #fff;
            text-align: center;
            position: relative;
            &:nth-child(2) {
              width: 100%;
              position: absolute;
              top: 0;
              left: 0;
              opacity: 0;
              transition: 0.3s all ease;
            }
          }
        }
        .accordion__icon {
          width: 1em;
          height: 2px;
          display: block;
          position: absolute;
          right: 1em;
          top: 50%;
          margin-top: -1px;
          &> div {
            background: #fff;
            width: 100%;
            height: 100%;
            position: absolute;
            top: 0;
            left: 0;
            transition: 0.4s ease;
            &:last-child {
              transform: rotate(90deg);
            }
          }
        }
        &:hover{
          background: #666;
        }
      }
    }
    &> input[type="checkbox"] {
      display: none;
      &:checked{
        &+ label {
          &+ .accordion__content {
            height: auto;
            mask-image: linear-gradient(0deg, rgba(0, 0, 0, 1) 10%, rgba(0, 0, 0, 1));
          }
          &> .accordion__button {
            background: #999;
            color: #fff;
            &> p > span{
              &:nth-child(1) {
                opacity: 0;
              }
              &:nth-child(2) {
                opacity: 1;
              }
            }
            &:hover {
              background: #666;
              color: #000;
            }
          }
          .accordion__icon div:last-child{
            transform: none;
          }
        }
      }
    }
    &> .accordion__content {
      text-align: left;
      height: 8em;
      padding: 0.5em 1em;
      overflow: hidden;
      transition: 0.5s;
      display: block;
      mask-image: linear-gradient(0deg, rgba(0, 0, 0, 0) 10%, rgba(0, 0, 0, 1));
    }
  }
}