PURPOSE: To reduce a slip by increasing the dynamic friction factor of a contact face, eliminate a slip by utilizing the engagement of irregularities, and provide resistance against a load and a load change.

CONSTITUTION: Irregular grooves are provided on a pulley 39, a motor shaft 37, or a magnet idler 38. The irregular grooves are formed as finely as possible and perpendicularly to the rotation direction (slip direction). Rough machining is applied on the surfaces of the motor shaft 37 and the pulley 39, and trapezoidal grooves are provided as finely as possible on the rubber surface of the magnet idler 38.