window.onload = function(){
var c = 5; //ランダムに表示するiframe内URLの数
var ifm = document.getElementById('random');
var r = Math.floor( Math.random() * c );
var urls = new Array();
urls[0] = 'http://www.e-tsumami.com/btoc/knowledge/iframe/awabi.html';
urls[1] = 'http://www.e-tsumami.com/btoc/knowledge/iframe/ika.html';
urls[2] = 'http://www.e-tsumami.com/btoc/knowledge/iframe/unagi.html';
urls[3] = 'http://www.e-tsumami.com/btoc/knowledge/iframe/uni.html';
urls[4] = 'http://www.e-tsumami.com/btoc/knowledge/iframe/ebi.html';
ifm.src = urls[r];
}