window.onload = function(){
	//マウスオーバーさせる要素のclass名と接尾語の設定をオブジェクトで管理
	var conf = new Object();
	conf["className"] = "btn";
	conf["postfix"] = "_on";
	

	
	var imgNodeList = getElementsByClassName(conf.className);
	var node;
	for(var i=0,len=imgNodeList.length;i<len;i++){
		node = imgNodeList[i];
		node.originalSrc = node.src;
		//正規表現で検索で当てはまる拡張子を$1で参照
		node.rolloverSrc = node.originalSrc.replace(/(\.gif|\.jpg|\.png)/,conf.postfix + "$1");
		
		//あらかじめロールオーバー用の画像を読み込んでおく
		preloadImage(node.rolloverSrc);
		
		node.onmouseover = function(){
			this.src = this.rolloverSrc;
		};
		node.onmouseout = function(){
			this.src = this.originalSrc;
		};
	}
	
	
		



	

	
	
}

		function getElementsByClassName(name){
			var elements = [];
			//全ての要素ノードを配列に格納
			var allElements = document.getElementsByTagName("*");
			for(var i=0,len=allElements.length;i<len;i++){
				if(allElements[i].className == name){
					elements.push(allElements[i]);
				};
			};
			return elements;
		}

		preloadedImages = [];
		function preloadImage(url){
			var p = preloadedImages;
			var l = p.length;
			p[l] = new Image();
			p[l].src = url;
		}