In order to keep mechanisms related to opening and closing of a drain valve disc out of the influence of drain stored in a bowl, there are provided, at positions higher than a liquid level of the drain stored in the bowl, a pressure receiving member moving up and down due to pilot air pressure in a pilot chamber and for opening and closing the drain valve disc, a pilot valve seat for communicating the pilot chamber with the interior of the bowl, a pilot valve disc for opening and closing the pilot valve seat, an exhaust orifice for communicating the pilot valve chamber with the atmospheric air, and a lever tilted by upward or downward movement of a float and for opening and closing the pilot valve disc.
