An oil pump for an automatic transmission includes a pump body having a pump chamber, a pump cover disposed so as to oppose to the pump body, an inner rotor disposed in the pump chamber and driven by a driving force from a torque converter and an outer rotor meshed with the inner rotor. The pump body includes a pump suction port, a pump discharge port a first oil, passage for supplying oil from a oil pan to the pump suction port, a second oil passage for returning excess oil of a first regulator to the first oil passage and a third oil passage having an outlet port for returning excess oil of a second regulator to the pump suction port.