PURPOSE: To keep the shaft power of an impeller at a necessary minimum level, by providing check valves which are seated in their valve seats by their own weight when the pump is not in operation, and thereby controlling the gas pressure such that it may not become higher than the discharge pressure of the pump or lower than a necessary level.

CONSTITUTION: As an impeller 4 is rotated, a space S1 is shifted to S2, S3 and so on, successively. At the position of the spaces S2, S3, pressure of the gas that has been enclosed in space S1 is raised gradually. In a space S5, for instance, a valve body 8a of a check valve 8 is urged by the gas pressure and discharges a portion of the gas in space S5 toward an outlet port 9. Similarly, gas is released to the outside from check valves 8', 8", successively. Thus, gas is released to the outside successively from check valves 8, 8', 8" and so on, the valve body 8a of which is fitted in its valve seat 8b only by its own weight, so that it is prevented that the gas pressure becomes excessively higher than the discharge pressure of the pum or lower than the necessary level. Therefore, the shaft power of the impeller shaft 6 can be kept at a necessary minimum level.