is fixed to an intermediate gear shaft 33 ba supported inside the casing 3 a so as to be parallel to the clutch shaft 32 a the intermediate gear 33 b is constituted by concentrically stacking and unitizing a large diameter gear 33 bb and a small diameter gear 33 bc having a diameter smaller than that of the large diameter gear 33 bb the large diameter gear 33 bb of the intermediate gear 33 b meshes with the output gear 33 a and a small diameter sector gear 33 cc of the driving gear 33 c described later the small diameter gear 33 bc of the intermediate gear 33 b meshes with a large diameter sector gear 33 cb of the driving gear 33 c the driving gear 33 c is fixed to the driving shaft 33 ca supported in the casing 3 a so as to be parallel to the clutch shaft 32 a and the intermediate gear shaft 33 ba the driving shaft 33 ca extends toward a front face of the casing 3 a the driving gear 33 c is constituted by concentrically stacking and unitizing the large diameter sector gear 33 cb and the small diameter sector gear 33 cc having a diameter smaller than that of the large diameter sector gear 33 cb the large diameter sector gear 33 cb of the driving gear 33 c is formed in a fan shape having teeth on an arc face and meshes with the small diameter gear.