To prevent the change of input and output speed ratio as much as possible in a device having a transmission mechanism having a stepwise transmission and a continuously variable transmission serially arranged on the output side of a driving force source by providing a speed ratio control means for changing the speed ratio of the continuously variable transmission in the gear shifting transition of the stepwise transmission.

A transmission accelerator 2 provided on the output side of an engine 1 has a transmission mechanism 3 and a final gear 4, and the transmission mechanism 3 has a stepwise transmission 8 in the power transmitting passage between an input shaft 5 and a counter shaft 6 and a continuously variable transmission 9 in the powder transmitting passage between the counter shaft 6 and an output shaft 7. An electronic control unit(ECU) is provided in order to control the stepwise transmission 8 and the continuously variable transmission 9, and the ECU controls the speed ratio of the continuously variable transmission 9 in the shifting of the stepwise transmission 9 so as to minimize the change quantity of the input and output ratio showing the ratio of the input-side rotating speed of the transmission mechanism 3 to the output- side rotating speed of the transmission mechanism 3.