To provide a pump improved in efficiency at a low cost while lowering the disk frictional loss with the simple structure in response to a request for miniaturization and high-efficiency of pump, comparing with a conventional one with the structure that as the number of revolution and diameter of impeller are increased for high lifting, the disk frictional loss is remarkably increased and the efficiency is lowered.

This pump is provided with an impeller 1 for giving the kinetic energy to the fluid, a casing 4 for housing the impeller 1, a shaft 2 and a bearing 3 for rotating the impeller 1, and a rear surface shroud 5 for the impeller 1. The bell-shaped opening side of the shroud 5 is directed downward so that the gas stays in a recessed part of the bell-shape.