An efficient use of discharged energy of a boom cylinder is provided. A control unit has a function of receiving an output signal from a sensor provided in a boom operated valve and determining a manipulated direction and a manipulated variable of the boom operated valve, and a function of, in accordance with the manipulated direction and the manipulated variable, controlling the degree of opening of a proportional solenoid valve and operating the tilt-angle control units to control the tilt angles of a sub pump and an assist motor. An output of the assist motor is used to assist in an output of the sub pump and an output of the electric motor.