PURPOSE: To save the fuel consumption by increasing a DC intermediate voltage for driving an inverter, obtained by rectifying AC output an engine-driven generator, from a low constant level according to the vehicle speed and switching the inverter output from VVVF to CVVF when a constant speed is reached.

CONSTITUTION: AC output of a generator 2 driven through an engine is rectified 3 to produce a DC intermediate voltage (Ed). The DC voltage Ed is applied on inverters 8L, 8R to produce AC three-phase voltage which is applied onto induction motors 9L, 9R for rotating right and left wheels. The voltage Ed is kept at a low constant level until an intermediate constant vehicle speed is reached after starting and then the voltage Ed is increased according to the vehicle speed. When a predetermined final vehicle speed is reached, the voltage Ed at that time is sustained. The inverters produce VVVF power before the final vehicle speed is reached and switching is made to production of CVVF power upon reaching the final vehicle speed. Semiconductor switches 7A, 7B are provided in series with two sets of brake resistors 6A, 6B in order to perform electric braking stably according to the vehicle speed. According to the invention, fuel consumption can be reduced.