var mainimage = new Array();

/* 使用する背景画像を好きなだけ記述 */
mainimage[0] = "img/mainimage01.jpg";
mainimage[1] = "img/mainimage02.jpg";
mainimage[2] = "img/mainimage03.jpg";
mainimage[3] = "img/mainimage04.jpg";
mainimage[4] = "img/mainimage05.jpg";
mainimage[5] = "img/mainimage06.jpg";
mainimage[6] = "img/mainimage07.jpg";
mainimage[7] = "img/mainimage08.jpg";
mainimage[8] = "img/mainimage09.jpg";
mainimage[9] = "img/mainimage10.jpg";

rnd = Math.floor(mainimage.length*Math.random());
mainimage = mainimage[Math.floor(rnd)];
document.write ('<style type="text/css"><!--#head {background: url("'+ mainimage +'") no-repeat #ccc;}--></style>');