PURPOSE: To reduce the occurrence of a pressure pulsation by a method wherein a plurality of holes are formed in the side wall surface of a flow passage between the vanes of a diffuser and intercommunicated through a bypass.

CONSTITUTION: A flow flowing in from a pump suction casing is boosted through rotation of an impeller and discharged to a diffuser. Holes 8 are formed in a side wall 5 surface between diffuser blades and intercommunicated through a groove 9 (a bypass pipe). Thus, even when a flow flowing out from an impeller is interfered at the front edge of the diffuser and pressure pulsation occurs, pulsations between which a phase is displaced are offset with each other through propagation of it through the groove 9 (a bypass pipe) from the holes 8 and a pressure pulsation in a diffuser flow passage is reduced. Especially, when a hole on the diffuser blade inlet negative pressure surface (a recessed surface) side where a pressure pulsation is highest is communicated with a hole in the same position between different blades between which a phase is displaced, a pressure pulsation is sharply reduced.