A regulator that performs control such that a tilt angle of a variable volume pump increases as a pilot pressure in a pilot flow passage decreases is provided, and when a controller determines that all of a plurality of operation valves for controlling a flow of a discharged oil that is led from the variable volume pump to respective actuators are in a neutral position, the controller supplies the discharged oil from the variable volume pump to a regenerative hydraulic motor by setting a solenoid valve in an open position and sets a pilot flow passage change-over solenoid valve in a blocking position such that the pilot pressure led to the regulator through the pilot flow passage is reduced by a pressure reducing valve.