A primary drive gear 10 is configured so as to have its position changed, by mounting the primary drive gear 10 in an axial direction within a length range of a crankshaft so as to displace a toothed portion 12 of said primary drive gear 10 in the axial direction of said crankshaft in response to a boss portion 13 integrally provided on said toothed portion 12 of said primary drive gear 10 so that said primary drive gear 10 is registered with a position in the axial direction of the primary driven gear meshing with said primary drive gear 10.