body {
    text-align: center;
    width: 100%;
    background:url(../image/common/bak_img_001a_C900.gif) center;
}
#div_container {  
position: absolute;  
left: 50%;  
height: 100%;  
width: 900px;  
margin-left: -450px; /* MUST be half the width */  
}