Try this...
.sub_menu {
background-image: url("../images/background_sub_navi.png");
background-position: center center;
height: 44px;
margin-top: -5px;
position: relative;
}
".sub_menu div" is ul's parent div which is text align center.
.sub_menu div{
position: absolute;
text-align: center;
width: 100%;
}
.sub_menu_arrow{
background-image: url("../images/sub_menu_border_active.png");
background-position: center 17px;
bottom: -4px;
display: block;
height: 19px;
left: 50%;
margin-left: -24px;
position: absolute;
width: 160px;
}