Abstract: An improved power transmission system for use with a light saddle-type vehicle with an internal combustion engine of cross-mounted type has an improved construction such that there are provided a split type crank case adapted to be split generally centrally into two counterparts in the vertical way, an output shaft mounted on the bottom of and extending along a splitting line of the crank case, and a bevel gear transmission disposed intermediate the output shaft and a secondary drive shaft so that the driving power on the secondary drive shaft from the engine may be transmitted operatively to the output shaft through the bevel gear transmission, and/or that there is provided an auxiliary transission mounted outwardly of an outer bearing means of the transmission housed in the case, and adapted to provide a substantially large overall gear ratio making an increased stage of gear shifting available in the operation of the vehicle, whereby the maneuverability of the vehicle can be improved substantially.