PURPOSE: To effectively restrain oscillation of a pump generated at the time of pumping cutoff or low output drive execution.

CONSTITUTION: A hollow opposed member 11 with a hollow part 11A of a roughly cut head conical section opposing to an impeller boss 8A of a pump P is arranged, and an air supply passage 13 directing an air supply outlet 13A in the tangential direction of the hollow part 11A is provided so as to swirl the air or the air mixed water in the hollow part 11A. The air discharged from the air supply outlet l3A is split into small bubbles 17, 17... from an outlet 11 of the hollow opposed member 11 and flown in between blades 8B, 8B... of an impeller 8 while the small bubbles 17, 17... are dispersed uniformly in petals.