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

CONSTITUTION: On the lower end of an impeller boss 8A of a pump, a cylindrical body 11 rotating simultaneously is provided, a plural number of collars 12, 12... overhanging in the diametrically outer direction with specified intervals are provided in the axial direction of this cylindrical body 11, small clearances 13, 13... are formed, a plural number of through holes 14 are formed on the peripheral wall of the cylindrical body 11 corresponding to the clearances 13, 13..., and from an outlet 15A of an air supply passage 15 open to the inside of the cylindrical body 11, air or air mixed water is discharged. It is devised so that when this discharged air passes a plural number of the through holes 14 which are rotating and while the discharged air flows out in a plural number of the clearances 13, 13... from the through holes 14, the discharged air is split in small bubbles 20, 20..., and thereafter, these small bubbles 20, 20... are uniformly dispersed and flow in between blades 8B, 8B... of an impeller 8.