A casing of a power unit is formed which is capable of simultaneously driving the front and the rear wheels of a saddle type vehicle. The power unit includes an engine and a transmission in a compact construction. A crankshaft, a main shaft, a secondary shaft and an output shaft are supported in parallel to the longitudinal axis of the vehicle in a casing for both an engine and a transmission. The crankshaft, the main shaft and the secondary shaft are arranged on substantially the same level. The output shaft is disposed below the main shaft and the secondary shaft. The opposite ends of the output shaft project outside from the casing. The opposite ends of the output shaft are connected to a front driving shaft and a rear driving shaft, respectively.