var boxIDs = ["navi"]; 　// ここに画像を切り替えるボックスのidを入力

window.onload = setchImg;

var imgsON = new Array();
var imgsOFF = new Array();
var btns = new Array();

function setchImg(){
    for(i in boxIDs){
	chImg(boxIDs[i]);
    }
}

function chImg(boxID) {   
    btns = getBtnsObj(boxID);
    preLoadBtImg(btns);
    setBtEvent(btns);
}

function getBtnsObj(str) {
    var q = new Array();
    q = document.getElementById(str).getElementsByTagName("img");
    return q;
}

function preLoadBtImg(arr) {
    var imgSrc;
    for(var i=0;i<arr.length;i++) {
	imgSrc = arr[i].src;
	imgsOFF[i] = new Image();
	imgsON[i] = new Image();
	imgsOFF[i].src = imgSrc;
	imgsON[i].src = imgSrc.replace('.gif','_om.gif');
    }
}

function setBtEvent(arr){
    var f = new Object();
    var g = new Object();
    for(var i=0;i<arr.length;i++) {
	f[i] = new Function("this.src = imgsON[" + i + "].src;");
	g[i] = new Function("this.src = imgsOFF[" + i + "].src;");
	arr[i].onmouseover = f[i];
	arr[i].onmouseout = g[i];
    }
}