their both axial side ends however since the stoppers 15a 15b are rotated in synchronism with the rotor 4 the aforesaid sliding amount is small and thus the lowering of the efficiency and the advance of the wear resulting from sliding resistance and sliding heat generation can be minimized and the temperature of the fluid discharged from the discharge port 24 can be lowered in addition according to the aforementioned arrangement since the stoppers 15a 15b which define the protrusion of the vanes 11a 11b and 11c are very close to the inner peripheral surface 1 34 of the housing the locus of the end edges 11a 39 11b 39 and 11c 39 of the vanes is approximately circular in shape despite the repeated change of the relative angle between the vanes 11a 11b and 11c and the inner peripheral surface 1 34 of the housing and the vanes rotate always leaving a given fine clearance in a state not in contact relative to the inner peripheral surface 1 34 of the housing while in the above described embodiment the cam is used to rotate the retainer plates in synchronism with the rotor it is noted that similar effects may be obtained by an arrangement wherein the retainer plates are rotated approximately in synchronism with the rotor by the frictional force between the vanes and the stoppers in addition while in the above described embodiment the stoppers are annularly formed it is noted that in the case where the retainer plates are rotated in synchronism with the rotor by the cam portions of the stoppers in contact with the vanes are restricted and therefore the stoppers can be formed in the form of an arc corresponding to those portions next a.