To alter a target gear ratio to a new shiftable target gear ratio automatically, in the case where a down-shift to this selected traget gear ratio is not achievable.

An input shaft 26 of a mechanical transmission 12 is connected to a fuel control engine 14 by a master friction clutch 16, and an output shaft 18 is connected to a vehicle driving axle. A fuel control unit 38, clutch operator 42 and a transmission operator 46 are controlled by a controller 54, shifting the transmission 12 automatically. In a down-shift to the selected target gear ratio, in the case where the detected engine speed is not yet reached to the synchronous engine speed at the target gear ratio after the elapse of the specified time, such a lower mode target gear ratio that maximum engine speed detected during the down-shift trial is made so as to be more than the synchronous engine speed at the current output shaft speed, is determined, whereby the down-shift to the lower mode target gear ratio is performed.