PURPOSE: To improve characteristics of fuel consumption while driving torque is being secured by providing an auxiliary transmission on the driving shaft at the rear wheel side which is driven at all times after switch-over between four-wheel drive and two-wheel drive is made, and thereby controlling switch-over between a large and a small shifting ratios.

CONSTITUTION: A transfer gear 4 with both a switch-over mechanism between two-wheel drive and four-wheel drive and a center differential switch-over mechanism is mounted on the rear section of a main transmission 3. A auxiliary transmission 13 is also provided between a propeller shaft 11 connected with an output shaft 7 at the rear wheel side 6 of the transfer gear 4 and a rear wheel differential mechanism 12 so that the output from the ordinarily driven output shaft 7 of the transfer gear 4 can be transferred to rear wheels 6 via the auxiliary transmission 13 and the differential mechanism 12. This constitution allows the auxiliary transmission 13 to be interlocked with the switch-over actuation of the switch-over mechanism between two-wheel drive and four-wheel drive so that switch-over can be controlled in such a way that the shifting ratio is switched-over to economy mode at the time of two-wheel drive and to power mode at the time of four-wheel drive.