A rotor rotates within a cam ring secured to the pump housing. There are grooves in the rotor and a vane plate built into the grooves. The outer circumference of the rotor is circular but the inner surface of the cam ring is oval so there exists a clearance between the rotor and cam ring. The vane plate partitions off this clearance to form a fluid chamber. The vane plate is held against the inner surface of the cam ring by both centrifugal force and fluid pressure against the back of the vane plate, forming a seal so that when the pump produces fluid pressure, pressure leakage from between the vane plate and cam ring is prevented. The capacity of this fluid chamber is increased or decreased as the rotor rotates to operate the pump. In other words, the capacity of the fluid chamber increases at the suction port so that reservoir fluid is drawn into the fluid chamber from the suction port. The volume of the fluid chamber is decreased on the discharge side, and when it reaches zero, the fluid previously drawn into the chamber is forced out through the discharge port. There are two suction and two discharge ports. Therefore, fluid is drawn in and discharged twice with each revolution of the rotor.