To provide an oil pump capable of sufficiently reducing generation of an abnormal sound, while minimizing generation of friction.

A driving shaft 6 having a first trochoidal pump 4, a second trochoidal pump 5 and a spacer 3 inside a housing body 1 and driving both trochoidal pumps, is driven by a helical gear 7. A falling-off preventive mechanism 62 is arranged for preventing slipping-out in the pump cover 2 direction of the driving shaft 6. The area of a delivery port 21b arranged in a pump cover 2 contacting with a second inner rotor 5a, is constituted larger than the area of a delivery port 31b of the spacer 3 becoming the opposite side.