To simplify a structure of a pump while reducing loss of power, to improve manufacturing efficiency, and to reduce a manufacturing cost.

A pumping action is performed by obtaining a volume change of a volume chamber 5 between inner and outer gears 3a and 4a of an inner rotor 3 and an outer rotor 4 which rotate by a drive shaft 2 with the gears of both rotors meshed. First and second discharge passage 15 and 16 are connected to two discharge ports 10 and 11 on a pump casing 1, and a 3-port 2-position type selector valve 19 is provided to appropriately switch to/from a relief passage 18 connected to the second discharge passage 16, or upstream and downstream parts 16a and 16b of the second discharge passage 16.