A pulley is concentrically disposed around a hollow shaft, and a one-way clutch is disposed between the hollow shaft and the pulley. At least one bearing is provided between the shaft and the pulley. In the one-way clutch, a roller as a meshing member and a coil spring as a biasing member are integrally rotated together with an outer ring. The roller moves in a meshing-releasing direction by a centrifugal force when a rotation speed of the outer ring becomes equal to or higher than a predetermined speed, and a biasing force of the coil spring has enough magnitude for permitting this movement. The pulley is provided at its inner periphery with a cam surface, and the pulley and the outer ring of the one-way clutch are integrally formed.