var imgArray = [
	'/sapporo/img/img_map.jpg',
	'/sapporo/img/img_map01.jpg',
	'/sapporo/img/img_map02.jpg',
	'/sapporo/img/img_map03.jpg',
	'/sapporo/img/img_map04.jpg',
	'/sapporo/img/img_map05.jpg'
];
var preloadImages = [];

for(var i=0,len=imgArray.length; i<len; i++) {
		preloadImages[i] = new Image();
		preloadImages[i].src = imgArray[i];
}

function changeOver(num) {
	var targetNode = document.getElementById('main');
	targetNode.src = preloadImages[num].src;
}