To make the size in an axial direction small by disposing at least one end of a bearing inside the inner surface (rotor side surface) of an annular plate part of a side plate.

A rotor 12 rotating slidingly on the inner surface of a cylinder 15 with air pressure on vanes 16 which are retractable in a radial direction and an output shaft 11 integrally rotating with the rotor 12 are disposed in the cylinder 15. Side plates 13, 14 are attached to both sides of the cylinder 15. Annular grooves 27, 28 are made in both ends 12b, 12c of the rotor 12. The side plates 13, 14 have annular plates 13a, 13b, which are in contact with end surfaces 15b, 15c of the cylinder 15 and the end surface 11c of the rotor 11, and sleeve parts 13b, 14b extending to the annular grooves 27, 28 in an axial direction. Bearings 17, 18 are disposed between the inner peripheral surface of the sleeve parts 13b, 14b and the outer peripheral surface 11c of an output shaft 11 and in recessed position in the annular grooves 27, 28.