the port member 12 which is opposite to the commutator 10 is formed with seven elongated grooves 44 which are arranged at equal spacing along the same circumference around the axis of the eccentric circular cam 18 and these elongated grooves 44 are connected to the other side of the port member 12 through holes 46 an annular groove 48 is similarly formed concentrically with the shaft center on the inner side of the grooves 44 and the groove 48 is also connected to the other side of the port member 12 through a hole 50 an elongated elliptic groove 52 which is circumferentially curved about the center of the shaft on the outer side of the diamondshaped grooves 44 is also connected to the other side of the port member 12 through a hole 54 the gerotor unit comprises the stator 24 a rotor 56 and a drive shaft 58 and five round bars 60 and hollow bushings 62 and 64 are fitted in the stator 24 thus forming seven internal teeth thereon the holes of the hollow bushings 62 and 64 constitute oil inlet and outlet passages and their positions respectively communicate with the hole 54 of the port member 12 and the hole 50 of the port member 12 the rotor 56 is formed with one less teeth than the number of teeth of the stator 24 and meshes with the internal teeth of the stator 24 the rotor 56 which is in mesh with the internal teeth of the stator 24 rotates about the center of the.