PURPOSE: To reduce the rotational load while increasing the rotational speed so as to reduce hydraulic pressure pulsation by forming a recess in a cam surface within an area which is coincident with the timing of lowering the pressure of hydraulic pulsation, and by eliminating a part of a vane pressing chamber facing the recess.

CONSTITUTION: When a rotor 108 is rotated inside of a cam ring 102 in which a cam 104 is formed at the inner surface thereof, a plurality of vanes 114 are extended and retracted in radial slits 110 formed in a rotor 108 so as to slide on the cam 104. As a result, the volumes of pump chambers defined between the vanes 114 are increased and decreased, effecting pump action. In this arrangement, a recess 106 is formed in the cam 104 at a position which is coincident with the timing of lowering the pressure of hydraulic pulsation. Further, in a vane pressing chamber 116 which introduces discharged oil to the inner end of each slit 110 so as to press the associated vane 114 against the cam 104, only a part 118 located radially inward of the recess 106 is eliminated.