PURPOSE: To maintain constant the ratio of driving force between front and rear wheels by matching the slip rate variation characteristic of an induction motor to that of an engine drive wheel with respect to the pavement.

CONSTITUTION: Slip rate characteristic of an engine drive wheel with respect to the pavement is similar to that of an induction motor 22. When the ratio of driving force is set constant between the engine drive wheel and the induction motor, an internal-combustion engine 16 and an induction motor 22 having constant ratio of maximum driving force are prepared. A source frequency operating means 40 multiplies the slip rate of the induction motor 22 by a predetermined number which equalizes the slip rate for the maximum drive force of the engine drive wheel and the slip rate for the maximum drive force of the induction motor 22, thus settling the ratio of driving force between the engine drive wheel and the induction motor constant within a predetermined range. Since distribution of driving force to front and rear wheels 12a, 12b, 14a, 14b does not vary even upon variation of the slip rate due to traveling on a pavement having low frictional coefficient, start acceleration performance and deceleration performance can be improved.