To provide a synthetic resin pulley to prevent generation of any vibration and noise of a roller bearing while maintaining the strength and rigidity by a rib.

In the synthetic resin pulley in which an inner cylindrical part, an outer cylindrical part for belt guide which is separated from the inner cylindrical part in the radial direction and coupled by a disk part, and a synthetic resin body part having a plurality of ribs formed in the radial direction on the circumferential side surface of the disk part are integrated with an outer circumferential surface of an outer ring of the roller bearing, the number of the ribs 5 is set to the number avoiding the integral multiple and the integral multiple ±1 of the number of rolling elements 8 of the bearing 6.