document.observe('dom:loaded', function() {
    var addAjaxEvent = function() {
        var prefectureSelect = $('c_prefecture_id');
        var citySelect = $('c_city_id');
        var a = ajax.api({
            getPrefectureId: function() {
                return prefectureSelect.getValue();
            },
            prefectureCompleteEvent: function(cities) {
                ajax.updateSelect(citySelect, cities, '');
            }
        });
        prefectureSelect.observe('change', a.updatePrefecture);
    };
    addAjaxEvent();
});