The present invention is to drive a hydraulic pump and a generator motor by an engine, to electrically charge a battery by a generator action of the generator motor, and to drive the generator motor by electric power of the battery so as to assist the engine. A governor position is changed by operations of a potentiometer for accelerator and a mode selection switch so as to control rotation speed of the engine. On the premise of the above configuration, generator output is limited by a controller at the time of accelerating the engine until the rotation speed of the engine reaches set speed determined on the basis of the operations of the potentiometer for accelerator and the mode selection switch, and hence engine load is reduced so as to assist acceleration.