divlist = document.getElementById("header").getElementsByTagName("div");
for(i=0; i < divlist.length; i++) {
  if(divlist[i].className == "top_menu") {
    var lilist = divlist[i].getElementsByTagName("li");
    divlist[i].onmouseover = function() {
      this.getElementsByTagName("ul")[0].style.display = "block";
    }
    divlist[i].onmouseout = function() {
      this.getElementsByTagName("ul")[0].style.display = "none";
    }
  } 
}