function taxchg(per) {

    var p = document.itemform.price.value;

    if(isNaN(p)){
        alert("半角で入力してください。");
        document.itemform.price.value = "";
        document.itemform.tax.value = "";
        document.itemform.price.focus();
    } else if (p != "") {
      var t = p - Math.floor(p / (per / 100 + 1));
      
      document.itemform.tax.value = t;
      document.itemform.box.focus();
    }

    return(false);
  }