PURPOSE: To improve strength and to reduce noise of a transmission by transmitting driving force coming from an internal combustion engine to a second shaft externally fitted to a first shaft through a planetary gear train from the first shaft and further to a differential mechanism through a transmission gear train a third shaft and a final reduction gear train.

CONSTITUTION: Driving force coming from an internal combustion engine 4 is transmitted t a second shaft 26 through a planetary gear train 38 from a first shaft 24, to a third parallel shaft 28 through a transmission gear train 50 and to a driving force distribution mechanism (differntial mechanism) 94 through a backward final reduction gear mechanism 80. In this case, the second shaft 26 is formed in a hollow shape, idly fitted to the first shaft 24 and supported by a transmission case 44 together with the first shaft 24. Thus, a large reduction gear ratio is obtained without increasing the diameter of the gear of a final reduction gear train. And also the deformation of the transmission case is decreased and the supporting strength of the respective shafts is increased to suppress the generation of noise.