<div class="bigblock">
<div class="box"></div>
</div>

.bigblock {
	position: relative;
}

.box {
  height: 300px;
  width: 300px;
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  margin: auto;
  overflow: auto;
}