<p>A drainage pump (10) in which the portion near the entrance of impeller (12) in the suction tube (11) opens to the set lowest suction water level (L.W.L.) through an air intake pipe (15), so that air is introduced into the portion (13) near the entrance of impeller (12) when the suction water level is lower than the preset lowest suction water level (L.W.L.). <IMAGE></p>
