Abstract: In a vane pump, the cam ring must be fixed to an annular recessed portion of the housing in such a way that there exists no eccentricity between the cam ring and the rotor. To eliminate an eccentricity, a pair of semicircular grooves are formed on the inner circumference of the annular recessed portion diametrically opposed to the center thereof; a pair of semicircular grooves are formed on the outer circumference of the cam ring also diametrically opposed to the center thereof so as to provide two locating pin holes in cooperation with the two housing grooves, respectively; a pair of locating pins are pressure fitted, respectively, to the locating pin holes. Further, it is possible forcibly correct an eccentricity by selecting a locating pin having a diameter appropriately larger than that of the pin hole. The two locating pin holes are arranged in the direction of belt tension to eliminate the eccentricity due to belt tension.