PURPOSE: To prevent the generation of cavitation, etc., by suppressing the exfoliation of the water stream in the vicinity of a suction port part by installing a tongue part which has a smooth water introducing curved surface for introducing at least a portion of the swirl stream to the most upstream side part in a swirl stream region, on the downstream side part of the swirl stream region on the periphery of a suction port part.

CONSTITUTION: In the prerotation type centrifugal pump in which the water sucked by the revolution drive of a blade wheel 3 forms a swirl stream in a flow passage in the vicinity of the eye part of a blade wheel, and approaches a blade wheel chamber 12, and sucked into the blade wheel chamber 12 through a suction port part 14, and direction change to the direction parallel to a pump shaft 2 is performed, a tongue part 5 for introducing the swirl stream which arrives at the downstream side part S1 to the blade wheel chamber 12 through the suction port part 14 is formed on the downstream side part S1 of a swirl stream region S. The tongue part 5 is formed out of the smooth water introducing curved surface 51 which introduces at least portion F1 of the swirl which arrives at the downstream side a part S1 of the swirl stream region S to the most upstream side part S2 of the swirl stream region S. Accordingly, the exfoliation phenomenon of the water stream in the suction port part 14 is suppressed.