PURPOSE: To reduce surge pressure and oil pressure pulsation without reducing pump displacement by providing a damping chamber adjacent to a pump chamber, and communicating the damping chamber with the pump chamber through an orifice while receiving a variable volume elastic member in the damping chamber.

CONSTITUTION: A damping structure is provided with a damping structure which is composed of a damping chamber 16, orifice 17 and elastic member 15. When a plunger 8 shifts from the suction stroke for opening a suction port 12 near the lower dead point to the compression stroke for closing the suction port 12 with the opening edge of a cylinder 7, the pump chamber 9 is tightly closed to abruptly increase the pressure therein. Then, differential pressure is generated between the pump chamber 9 and damping chamber 16 and working oil flows gently through the orifice 17 into the damping chamber 16, so that the pressure in the pump chamber 9 is restrained from the abrupt rise. Hence, surge pressure generated immediately before the opening of a check valve 20 is kept extremely low and oil pressure pulsation is also reduced immediately after the opening of the check valve 20.