A transmission device in which a centrifugal starting clutch is in a casing mounted on a crank shaft of an engine. A multiple plate type speed change clutch is mounted on a speed change input shaft. The starting clutch has an output member operatively connected to its input member through a reduction gear train. The speed change clutch is interposed between the starting clutch and the reduction gear train. The starting clutch is immersed into lubricating oil in the casing, whereas the speed change clutch is totally positioned above the surface of lubricating oil.