PURPOSE: To prevent abnormal high pressure and cavitation at a closing part from occurring, by installing a groove, whose both ends are interconnected to each closing part at both suction and discharge sides of a cam ring inner circumferential surface, at the side of a cam ring, while making each width of both ends of the groove smaller than thickness of a vane.

CONSTITUTION: At the side of a cam ring 14, there is provided with a groove 50 extending over the semicircumference, and both ends of which extends as far as the radial inner part, reaching an inner diametral part of the cam ring 14. Both these ends 50a and 50b of the groove 50 are interconnected each to a suction side closing part 60 and a discharge side closing part 62, while width lat these ends of the groove 50 is set to be smaller than thickness (t) of a vane 18. With this constitution, cavitation at the suction side closing part 60 and abnormal high pressure at the discharge side closing part 62 both are preventable with a simple structure, while when the vane 18 passes through both these ends 50a and 50b of the groove 50, before and behind parts of the vane 18 are blown through, thus any drop in pump efficiency is preventable.