var ul = document.getElementById("ul_nav");
var li = ul.getElementsByTagName("li");

for(i=0; i<li.length; i++){
li[i].ids = i;


li[i].onmouseover  = function(){	
		for(j=0; j<li.length; j++){
			li[j].className = "";
		}
		li[this.ids].className = "li_now";
	}

}



/*var ul = document.getElementById("ul_nav");
var li = ul.getElementsByTagName("li");

for(i=0; i<li.length; i++){
li[i].ids = i;

li[i].onmouseover = function(){
		var m=this.ids;
		li[m].className = "li_now";
		}
li[i].onmouseout = function(){
		var m=this.ids;
		li[m].className = "";
		}

}*/