function rand_bg () {
	types = new Array(3);

	types[0] = "type1";
	types[1] = "type2";
	types[2] = "type3";
	types[3] = "type4";
	

	//配列をシャッフル
	for (i=0 ; i<types.length; i++) {
		var tmpA, tmpB, rnd;
		rnd = Math.floor(Math.random() * types.length);
		tmpA = types[i];
		tmpB = types[rnd];
		types[i] = tmpB;
		types[rnd] = tmpA;
	}
	
	Element.addClassName("type", types[0]);

}