The output of an engine driven generator G is directly supplied to a motor 7 for travel without the intermediary of a battery. The output capacity of each of the generator G and motor 7 is determined so that the maximum output of the generator G may substantially coincide with the maximum control electric power that is supplied to the motor 7. The over-current with respect to the motor is suppressed by the output impedance of the generator G. A CPU 102 performs output control so as for the output to conform to the load according to the electric current with respect to the motor. If the electric current increases, the voltage is varied to a level thereof that conforms to that electric current for ensuring sufficient torque.