PURPOSE: To moderate a sudden pressure variation by releasing the pressure in a pump chamber to a discharge port through a space part which is dividedly formed by a chamfering part by forming a chamferred part at a part at least on one peripheral surface among the outer and inner rotors which dividedly form a pump chamber.

CONSTITUTION: As for an oil pump 2, an inner rotor 6 and an outer rotor 12 are accommodated in rotatable manner inside a case 4 so that gear parts 8 and 10 are meshed. Further, on the case 4, a suction port 16 and a discharge port 18 which communication to a pump chamber 14 are formed. Further, a plate 20 for preventing the slip of both the rotors 6 and 12 is installed. In this case, a specified tip clearance A is formed between both the rotors 6 and 12, and each side clearance B1, B2 is formed between both the rotors 6 and 12, case 4, and the plate 20. Each chamferred part 22 is formed at the edge parts of both the rotors 6 and 12, and the space parts 24-1 and 24-2 are dividedly formed on the case 4 side and the plate 20 side.