PURPOSE: To prevent abrasion dust emerged due to friction of a tooth part of a toothed belt and a flange to prevent meandering of the toothed belt provided on a pulley from splashing on the side of a belt surface.

CONSTITUTION: An extruded part 6 extruded outward is provided by increasing the breadth of a back plate part 5 of a toothed belt 1 wider than a tooth part 3 of the toothed belt 1. A flange 7 which is narrower in breadth than the breadth of the extruded part 6 of the back plate part 5 is provided on the side edge of a pulley 2 on which this toothed belt 1 is wound around, and distance from the center of the pulley 1 to the outer periphery of the flange 7 is made equal to or slightly shorter than distance from the center of the pulley 2 to the tooth trough part of the toothed belt 1 at a position wound around on the pulley 2. Consequently, when the toothed belt 1 revolves, both sides of the tooth part 3 make contact with the flange 7 provided on the pulley 2 and meandering is prevented. Abrasion dust emerged from a belt material because of this contact is blocked by the extruded part 6 of the back plate part 5 and prevented from splashing on the surface side of the back plate part 5.