PURPOSE: To prevent loss of power and pressure rise of operating oil by diminishing the discharge quantity of a pump chamber when the load on a pump discharge side passage is small.

CONSTITUTION: A cam ring 3 is eccentrically rotatably arranged along the outer circumference of a rotor 1, and a pump chamber c is formed between the rotor 1 and the cam ring 3. An orifice 27 is interposed on a pump discharge side passage. A piston 9 energized in an initial regulation position by a spring 10 for return is connected to the cam ring 3. The piston 9 is operated in response to a differential pressure between those before and behind the orifice 27. A regulation position adjusting mechanism 31 which advances the initial regulation position of the piston 9 when orifice forward pressure becomes below the set value is provided. When the load on the pump discharge side passage is diminished and the orifice forward pressure drops below the set value, the regulation position adjusting mechanism 31 advances the initial regulation position of the piston 9, and eccentric quantities of the rotor 1 and the cam ring 3 are diminished. As a result, the discharge quantity of the pump chamber c is reduced.