window.onload = function(){
	var imgNodeList = getElementsByClassName('btn');
	var node;
	for(var i=0, len=imgNodeList.length; i<len; i++){
		node = 	imgNodeList[i];
		node.originalSrc = node.src;
		node.rolloverSrc = node.originalSrc.replace('.gif', 'a.gif');
		node.onmouseover = function(){
			this.src = this.rolloverSrc;
		};
		node.onmouseout = function(){
			this.src = this.originalSrc;	
		};
	}
};
//クラス名によるエレメントノード配列取得
function getElementsByClassName(name){
	var elements = [];
	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;
}