bgimg = new Array();
bgimg[0] = "img/index_bg/bg_01.jpg";
bgimg[1] = "img/index_bg/bg_02.jpg";
bgimg[2] = "img/index_bg/bg_03.jpg";
bgimg[3] = "img/index_bg/bg_04.jpg";
bgimg[4] = "img/index_bg/bg_05.jpg";
bgimg[5] = "img/index_bg/bg_06.jpg";
bgimg[6] = "img/index_bg/bg_07.jpg";
bgimg[7] = "img/index_bg/bg_08.jpg";
bgimg[8] = "img/index_bg/bg_09.jpg";
bgimg[9] = "img/index_bg/bg_10.jpg";
bgimg[10] = "img/index_bg/bg_11.jpg";
bgimg[11] = "img/index_bg/bg_12.jpg";
bgimg[12] = "img/index_bg/bg_13.jpg";
bgimg[13] = "img/index_bg/bg_14.jpg";
bgimg[14] = "img/index_bg/bg_15.jpg";
bgimg[15] = "img/index_bg/bg_16.jpg";
bgimg[16] = "img/index_bg/bg_17.jpg";
bgimg[17] = "img/index_bg/bg_18.jpg";
bgimg[18] = "img/index_bg/bg_19.jpg";
bgimg[19] = "img/index_bg/bg_20.jpg";
chip = Math.floor(bgimg.length * Math.random());
document.body.background = bgimg[chip];