To prevent application of sudden tension fluctuation to a power transmission belt and to elongate the service life of the belt.

A one-way clutch and a bearing are disposed between a pulley and a shaft body. The one-way clutch comprises an outer ring member, an inner ring member, and rolling elements circumferentially rotatably received in plural wedge-shaped spaces formed as a cam face in the circumferential direction of at least one of an inner peripheral face of the outer ring member and an outer peripheral face of the inner ring member. The rolling elements are switchable between a locked state to be engaged with a narrow side of the wedge-shaped spaces and a free state to be disengaged therefrom to evacuate to a wide side of the wedge-shaped spaces. The pulley is made of a flexible material.