An output sprocket wheel attachment structure includes a coned disk spring fitted with an output power shaft is abutted with and positioned by a stepped portion. An output sprocket wheel spline-fitted with spline grooves formed at an outer side portion of the output power shaft with respect to the stepped portion is pressed by the coned disk spring. A cap nut which engages with male threads formed at an end portion of the output power shaft is restricted and fixed at a bottom face thereof by and to an axial end of the output power shaft. Movement in an axial direction of the output sprocket wheel pressed by the coned disk spring is restricted by an opening end portion of the cap nut.