PURPOSE: To restrain an amount of occurrence of a tip cavitation by chipping both corner parts between a tip and a front edge of each blade of an impeller, and forming a smooth curved surface.

CONSTITUTION: A clearance δ set up so as to make the impeller 7 rotates smoothly, and to get a fluid leakage minimized, is formed in an interval between a tip 10 of a blade 10 of impeller 7 and an inner circumferential surface of a wear ring 2. In addition, a square corner part between the tip 10a and a front edge 10b of the blade 10 is chipped, whereby a smooth curved surface 21 consisting of a circular arc of a radius R is formed, by way of example. A chip cavitation is produced in a clearance δ in the section of the blade 10 being lowest in static pressure, but since the square corner part between the tip 10a and the front edge 10b of the blade 10 is chipped and this curved surface 21 is therefore formed, an inlet of a fluid S to the clearance δ is an inlet sectional position of the actual blade 10, namely, it comes to a Q point (a position where static pressure is boosted as far as the constant value) after shifting to the downstream side from the blade front edge 10b. Accordingly, an amount of occurrence of the tip cavitation is restrained.