일반 3시간 걸려서 만든 입력폼 자릿수체크 스크립트

function validKoujiNo(obj) {
        var koujino = document.getElementById(obj);
        if(koujino.name=='koujino1' && koujino.value.length > 0 && koujino.value.length < 2) {
                alert('文字列を二つ入力してください（例：00,12,...）');
                koujino.focus();
        }
        if(koujino.name=='koujino2'&& koujino.value.length > 0 && koujino.value.length < 4) {
                alert('文字列を四つ入力してください（例：0000,1212,...）');
                koujino.focus();
        }
        if(koujino.name=='koujino3'&& koujino.value.length > 0 && koujino.value.length < 2) {
                alert('文字列を二つ入力してください（例：00,12,...）');
                koujino.focus();
        }
}

<INPUT type="text" id="koujino1" name="koujino1" value="" size="2" class="TextBox" onfocusout="javascript:validKoujiNo('koujino1')">
<INPUT type="text" id="koujino2" name="koujino2" value="" size="4" class="TextBox" onfocusout="javascript:validKoujiNo('koujino2')">
<INPUT type="text" id="koujino3" name="koujino3" value="" size="2" class="TextBox" onfocusout="javascript:validKoujiNo('koujino3')">