<script type="text/javascript">
function numbersonly(myfield, e, dec) {
  var key;
  var keychar;
  if (window.event)
    key = window.event.keyCode;
  else if (e)
    key = e.which;
  else
    return true;
  keychar = String.fromCharCode(key);
  // control keys
  if ((key==null) || (key==0) || (key==8) || (key==9) || (key==13) || (key==27) )
    return true;
  // numbers
  else if ((("0123456789").indexOf(keychar) > -1))
    return true;
  // decimal point jump
  else if (dec && (keychar == ".")) {
    myfield.form.elements[dec].focus();
    return false;
  } else
alert("İd Sadece Rakamlardan Oluşur Harf Ve Karakter Yazamazsınız"); 
    return false;
}
</script>

<input type="text" name="Mid" onkeypress="return numbersonly(this, event)" id="sadecerakam">