PURPOSE: To realize low noises by forming fixed inlet and outlet in the same shaft end of a gerotor pump provided with a ring and a rotor having different number of projections, and forming the projections of both members in helical shapes along a longitudinal direction.

CONSTITUTION: In a cylindrical cavity formed in a pump main body 10, a ring 12 rotated around a shaft 14 is supported, and a rotor 16 is attached to be rotated on a shaft 18. The rotor has five projections, the ring has six projections and, in the pump main body 10, an inlet 20 and an outlet hole 22 are formed in its end wall. In this case, the projections provided in the ring 12 and the rotor 16 are formed in helical shapes having fixed and identical angle slopes in longitudinal directions. Each of a helical angle is selected such that a phase difference between one end of each projection and the opposite end thereof is positioned within the range of +5 to +15 degrees. Thus, since a pressure is made smooth, a pump operation with little noise is performed.