To solve such problems with a pulley that the rigidity thereof is partially different in circumferential direction between a place where reinforcement ribs are provided and a place where the reinforcement ribs are not provided, and when teh pulley is thermally shrunk due to a variation in atmospheric temperature and rolling bearing temperature, an outer ring member is partially pressed through the reinforcement ribs and the outer ring member is deformed.

By extending the radial inner end parts of the reinforcement ribs 21 midway to a support part 11 without reaching a base part 20, the rigidity of the base part 20 is uniformized in a circumferential direction, the concentration of a load from the reinforcement ribs 21 thereon is eliminated, and thus the deformation of the outer ring member 5 by the load can be prevented. Even if resin is thermally shrunk due to a variation in the atmospheric temperature of a pulley device 1 and the temperature of radial bearings 2, such a problem that the outer peripheral surface of the outer ring member 5 is partially pressed against the reinforcement ribs 21 can be solved. As a result, the eccentric deformation of the outer ring member 5 can be prevented, and the lowering of roundness of the route surfaces of balls 6 in the outer ring member 5 can be prevented. Also the generation of vibration of the pulley device 1 in service and the occurrence of noise can be prevented.