To prevent engine stall or shock caused by a steep increase or decrease of the power generating force of a generator for supplying power to a motor driving a follower wheel.

A changeover switch 7 is provided to connect an alternator 8 coupled with an engine 1 selectively to either of the side with a battery 5 and load 6 and the side with a motor 4, and when a slip sensing means 12 has sensed a slipping condition, a control device 10 changes over the switch 7 to the motor side and controls the power generating amount of the alternator 8 in accordance with the degree of opening of a throttle valve 13. Because the control of the power generating amount when the power of the generator 8 is supplied to the motor 4 for driving the other wheel in order to get off from the slipping condition of one of the wheels coupled with the engine 1 is made in accordance with the degree of throttle opening, the power generating amount increases or decreases in compliance with the engine output and a steep load change for the engine 1 can be prevented, and it is possible to preclude engine stall and shock.