To provide a four-wheel driving gear and its driving method utilizing a motor.

This four-wheel driving gear for a vehicle whose front wheels are driven by an engine 11 includes a generator that generates a current by being connected to the engine, a motor that provides a driving force for driving rear wheels by the current supplied from the generator, a battery that is charged by the current supplied from the generator, a first relay that switches the contact point of the generator with the motor, and a second relay that switches the contact point of the generator with the battery. The current generated by the generator is supplied alternatively to the motor or the battery.