PURPOSE: To make a setting position between a pump head and a pump ring adjustable at will, by installing a radial engaging groove part, which is centering on a shaft and free of engagement in the specified turning angular range, in an end face where the pump head and the pump ring are superposed on each other.

CONSTITUTION: On an end face where a pump ring 2 is superposed on a pump head 1, there is provided with a projection part 21 of serrated one or two pieces slightly projected to a nearby part of a minimal clearance part between an inner circumferential surface of the pump ring 1 and an outer circumferential surface of a rotor 5. On the other hand, in the part opposed to the said projection part 21 on the end face of the pump head 1, there is provided with a radial groove part 23 which is centering on a shaft 4 and free of engagement with the projection part 21 in the specified turning angular range. Therefore, in the state that the pump ring 2 is superposed on the pump head 1, an engaging position between the projection part 21 and the groove part 23 is adjusted while shifting it in a circumferential direction whereby positioning for both is accurately performable.