function smartRollover() {
	if(document.getElementsByTagName) {
		var images = document.getElementsByTagName("img");

		for(var i=0; i < images.length; i++) {
			if(images[i].getAttribute("src").match("_n")){
				fileName = new Array(images[i].getAttribute("src").replace("_n.", "_a."));
				preImages = new Array();
				for (j=0; j<fileName.length; j++){
						preImages[j] = new Image();
						preImages[j].src = fileName[j]; //「_a」の画像をプリロード
				}
				
			images[i].onmouseover = function(){
				this.setAttribute("src", this.getAttribute("src").replace("_n.", "_a.")); //マウスオーバー_n→_a
			}
			images[i].onmouseout = function() {
				this.setAttribute("src", this.getAttribute("src").replace("_a.", "_n.")); //マウスアウト_a→_n
			}
			}
		}
	}
}

if(window.addEventListener) {
	window.addEventListener("load", smartRollover, false);
}
else if(window.attachEvent) {
	window.attachEvent("onload", smartRollover);
}
else{
window.onload = smartRollover;
}