function setCity(flag){
       var fm;
       var contry;
       var city;
       if(flag<=3){
        fm = document.form;
       }else{
        fm = document.form1;
       }
       if(flag=='0'){
        contry = fm.startContry.value;
        city = fm.startCity;
       }else if(flag=='1'){
        contry = fm.toContry.value;
        city = fm.toCity;
       }else if(flag=='2'){
        contry = fm.backContry.value;
        city = fm.backCity;
       }else if(flag=='3'){
        contry = fm.backToContry.value;
        city = fm.backToCity;
       }
       switch(contry){
       case '1':
        var labels = new Array("東京(羽田)","大阪(全て)","大阪(伊丹)","大阪(関西)");
        var values = new Array("東京(羽田)","大阪(全て)","大阪(伊丹)","大阪(関西)");
        break;
       case '2':
        var labels = new Array("上海(浦東虹橋)","上海(浦東)","上海(虹橋)","北京","広州");
        var values = new Array("上海(浦東虹橋)","上海(浦東)","上海(虹橋)","北京","広州")
        break;
       case '3':
        var labels = new Array("台湾(桃園松山)","台湾(桃園)","台湾(松山)","高雄");
        var values = new Array("台湾(桃園松山)","台湾(桃園)","台湾(松山)","高雄");
        break;
       case '4':
        var labels = new Array("香港");
        var values = new Array("香港");
        break;
       case '5':
        var labels = new Array("ソウル(仁川金浦)","ソウル(仁川)","ソウル(金浦)","釜山");
        var values = new Array("ソウル(仁川金浦)","ソウル(仁川)","ソウル(金浦)","釜山");
        break;
       }
       city.options.length=0;
       for(var i=0;i<labels.length;i++){
        city.add(document.createElement("option"));
        city.options[i].text = labels[i];
        city.options[i].value = values[i];
       }
       city.selectedlndex = 0;
      
      }
这个JS函数更完善，当一个页面有多个重复的下拉列表时候，而且他们的name都不同，可以通过传递参数来判断到底是哪一个下拉列表框，最后不要忘了在body结束后调用一下setCity(参数)；