PURPOSE: To improve workability of screwing of a drain tap and reliability of a seal for a self-priming pump.

CONSTITUTION: A protruded drain nozzle 31 with a male screw 32 on the outer dia. at the end, an O-ring 34 attached to the outer dia. at the bottom and a drain hole 33 at the center is provided at the bottom of the casing 21 of a pump. Also, a drain tap 35 which has a female screw 36 fitted to the male screw 32 of the drain nozzle 31 and a drain through-hole 37 smaller in dia. than the female screw 36 and has a plane packing 39 fitted in a level difference between the female screw 36 and the through-hole 37 so as to be held in a hollow with a rib 38 is provided.