PURPOSE: To prevent fault causd by residual dirt in an impeller by approximately matching the inner wall face of delivery port at the opposite side of suction port with the tangent of innercircumferential circle of casing while forming the inside opening face angularly.

CONSTITUTION: The pump casing 1 is provided with tubular inner face while a crescent cam 2 is provided integrally on the upper portion and formed such that a portion of circumference will project linearly. The suction port 1a and the delivery port 1b are made such that the inner wall face at the opposite side will match with the stangent of innercircumferential circle while the inside open face is formed angularly to provide a path which will become narrower toward the outer end. Consequently, the flow at the delivery port will be smooth to prevent the impeller lock.