To provide an oil pump apparatus having multiple or multistage oil pumps integrally constituted in a compact manner.
A first rotor 1 having an external gear 1a at the outer peripheral part is provided on the innermost side, and a second rotor 2 formed in ring shape and provided at the inner peripheral part with an internal gear 2a meshed with the external gear 1a of the first rotor 1, is eccentrically provided outside the first rotor 1. An external gear 2b is provided at the outer peripheral part of the second rotor 2, and a third rotor 3 formed in ring shape and provided at its inner peripheral part with an internal gear 3a meshed with the external gear 2b of the second rotor 2, is eccentrically provided outside the second rotor 2. A plurality of oil pumps are constituted by providing multiple rotors in a casing 10 to drive the first rotor 1, then the second rotor 2 meshed with the first rotor, and further the third rotor in this order.