PURPOSE: To prevent the application of a high-torque driven force only to rear wheels by the operation of one-way clutch by providing a direct coupler between the front and rear wheel's output shafts.

CONSTITUTION: Front wheel-driven shaft 4 and rear wheel-driven shaft 2 are connected through an one-way clutch 16, and a direct coupler 20 to make the one-way clutch inoperative is provided on the outer lace 17 of the one-way clutch 16. The shift fork 26 of the direct coupler 20 is operated interlockingly with the shift fork 15 of an auxiliary change gear, and when the auxiliary change gear is shifted toward low-speed side, the fornt wheel-driven shaft is directly coupled. Since a driving force is always transmitted to the front and rear wheels during the low-speed driving period, no application of great driving force only to the rear wheels occurs.