var div_scroll = document.getElementById('div_scroll');

function scroll_up() {
  scroll_up_t(18, 7, 10);
}
function scroll_down() {
  scroll_down_t(18, 7, 10);
}
function scroll_up_t(times, step, delay) {
  var y = div_scroll.scrollTop;
  y = y-step;  if (y < 0) y=0;
  div_scroll.scrollTop = y;
  times = times-1;
  if ((times <= 0) || (y <= 0)) return;
  window.setTimeout('scroll_up_t('+times+','+step+','+delay+')', delay);
}
function scroll_down_t(times, step, delay) {
  var y = div_scroll.scrollTop;
  y = y+step;
  div_scroll.scrollTop = y;
  times = times-1;
  if (times <= 0) return;
  window.setTimeout('scroll_down_t('+times+','+step+','+delay+')', delay);
}

if (div_scroll.clientHeight >= div_scroll.scrollHeight) {
  document.getElementById('arrow_up').style.display = 'none';
  document.getElementById('arrow_down').style.display = 'none';
}