To regenerate the power with an electric motor without generating an uneasy feeling in the operation of an engine brake.

A buffering clutch 5 having an adjustable connecting force is arranged on a power transmission system 3 common to an electric motor 1 and an internal combustion engine 2. While an engine brake is operated, the connecting force of the buffering clutch 5 is held in the partial clutch engagement condition, and the rotation of traveling wheels is transmitted to the electric motor 1 so as to regenerate the power. Slide of the buffering clutch 5 is utilized to prevent the transmission of the rotary resistance of the electric motor 1 to be increased by the regeneration of power to the traveling wheels, and the stabilized engine brake force can be obtained without generating torque fluctuation.