function preload(imgs){
 for(var i = 0; i < imgs.length; i++){
   var imgObj = new Image();
   imgObj.src = imgs[i];
 }
}
preload(["http://www.natureworld.jp/images/common/bt_air01-out.jpg", 
"http://www.natureworld.jp/images/common/bt_air01-over.jpg", 
"http://www.natureworld.jp/images/common/bt_air02-out.jpg", 
"http://www.natureworld.jp/images/common/bt_air02-over.jpg", 
"http://www.natureworld.jp/images/common/bt_tour01-out.jpg", 
"http://www.natureworld.jp/images/common/bt_tour01-over.jpg", 
"http://www.natureworld.jp/images/common/bt_tour02-out.jpg", 
"http://www.natureworld.jp/images/common/bt_tour02-over.jpg", 
]);

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

		for(var i=0; i < images.length; i++) {
			if(images[i].getAttribute("src").match("-out."))
			{
				images[i].onmouseover = function() {
					this.setAttribute("src", this.getAttribute("src").replace("-out.", "-over."));
				}
				images[i].onmouseout = function() {
					this.setAttribute("src", this.getAttribute("src").replace("-over.", "-out."));
				}
			}
		}
	}
}

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

//function sakiyomi(images){
//	for(var i = 0; i < images.length; i++){
//	var imgObj = new Image();
//	imgObj.src = images[i];
//	}
//}