.back_img {
  background: url(../img/story/bg_01.jpg) no-repeat center center;
  -webkit-background-size: cover;
          background-size: cover;
}

.back_img ul {
  position: relative;
  z-index: -1;
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
}

.back_img ul li {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  opacity: 0;
  filter: alpha(opacity=0);
  -webkit-transition: opacity .6s linear;
          transition: opacity .6s linear;
}

.back_img ul li.back01 {
  background: url(../img/story/bg_01.jpg) no-repeat center center;
  -webkit-background-size: cover;
          background-size: cover;
}

.back_img ul li.back02 {
  background: url(../img/story/bg_02.jpg) no-repeat center center;
  -webkit-background-size: cover;
          background-size: cover;
}

.back_img ul li.back03 {
  background: url(../img/story/bg_03.jpg) no-repeat center center;
  -webkit-background-size: cover;
          background-size: cover;
}

.back_img ul li.now_img {
  opacity: 1;
  filter: alpha(opacity=100);
}

@media screen and (max-width: 750px) {
  .back_img {
    background: url(../img/story/_sp/bg_sp01.jpg) no-repeat center center;
    -webkit-background-size: cover;
            background-size: cover;
  }
  .back_img ul li {
    height: 100vh;
  }
  .back_img ul li.back01 {
    background: url(../img/story/_sp/bg_sp01.jpg) no-repeat center center;
    -webkit-background-size: cover;
            background-size: cover;
  }
  .back_img ul li.back02 {
    background: url(../img/story/_sp/bg_sp02.jpg) no-repeat center center;
    -webkit-background-size: cover;
            background-size: cover;
  }
  .back_img ul li.back03 {
    background: url(../img/story/_sp/bg_sp03.jpg) no-repeat center center;
    -webkit-background-size: cover;
            background-size: cover;
  }
}