function cascadingDropDownList(parent, child, isSubselectOptional, childVal) {
  $("body").append("<select style='display:none' id='" + parent + child + "'></select>");
  $("#" + parent + child).html($("#" + child + " option"));

  var parentValue = $("#" + parent).attr("value");
  $("#" + child).html($("#" + parent + child + " .sub_" + parentValue).clone());

  childVal = (typeof childVal == "undefined") ? "" : childVal;
  $("#" + child + ' option[@value="' + childVal + '"]').attr("selected", "selected");

  $("#" + parent).change(function() {
    var parentValue = $("#" + parent).attr("value");
    $("#" + child).html($("#" + parent + child + " .sub_" + parentValue).clone());
    if (isSubselectOptional) $("#" + child).prepend("<option value='none'> -- 選択 -- </option>");
    $("#" + child).trigger("change");
    $("#" + child).focus();
  });
}