Edit in JSFiddle

<section class="scroll-btn">
    <span class="mouse">
        <span></span>
    </span>
</section>

body{
    background-color: #1d1d1d;
}

.scroll-btn {
  display: block;
  position: absolute;
  text-align: center;
  top: 20px;
  left: 20px;
  opacity: 1;
}

.scroll-btn .mouse {
  position: relative;
  display: block;
  width: 150px;
  height: 220px;
  margin: 0 auto 40px;
  border: 5px solid #fff;
  border-radius: 70px;
}

.scroll-btn .mouse span {
  position: absolute;
  display: block;
  top: 25%;
  left: 45%;
  width: 20px;
  height:20px;
  margin: -3px 0 0 -3px;
  background: #fff;
  border-radius: 50%;
  -webkit-animation: scroll 2s infinite linear;
  -moz-animation: scroll 2s infinite linear;
  animation: scroll 2s infinite linear;
}

@-webkit-keyframes scroll {

  0% { 
    opacity: 1;
    top: 40%;
  }

  33% {
    opacity: 1;
    top: 25%;
  }


  66% {
    opacity: 0;
    top: 25%;
  }


  100% {
    opacity: 0;
    top: 40%;
  }
}

@-moz-keyframes scroll {

  0% {
    opacity: 1;
    top: 40%;
  }


  33% {
    opacity: 1;
    top: 25%;
  }


  66% {
    opacity: 0;
    top: 25%;
  }


  100% {
    opacity: 0;
    top: 40%;
  }
}

@keyframes scroll {

  0% {
    opacity: 1;
    top: 40%;
  }

  33% {
    opacity: 1;
    top: 25%;
  }


  66% {
    opacity: 0;
    top: 25%;
  }


  100% {
    opacity: 0;
    top: 40%;
  }
}