<html>
<head>
<title>ほらよ</title>
<script>
function onClick()
{
var n, s;
n = parseInt(f.t.value, 10) + 1;
s = "00" + n;
f.t.value = s.substr(s.length - 3, 3);
}
</script>
</head>
<body>
<form name="f"><input type="text" name="t" value="000">
<input type="button" value="押してね" onclick="onClick()">
</form>
</body>
</html>