.bg_top {
  /* Set rules to fill background */
  min-width: 1000px;
  /* Set up proportionate scaling */
  width: 100%;
  /* Set up positioning */
  position: absolute;
  /* またはabsolute; */
  bottom:0;
  left:0;
  z-index:-100;
  background:url(../img/bg.jpg) #65d3ff left bottom no-repeat;
  height: 100%;
  background-size: 100% auto;
}