Effective use of energy is provided by recovering energy produced during braking in operation of a rotation motor RM alone for electric generation. A control unit C has functions: of operating a passage-resistance control unit (51) to reduce the passage resistance by a pressure relief valve 50 when determining based on a detection signal from a neutral-condition detecting unit (6, 8, 9, 11 and 16, 18, 19, 21) that all the operated valves 1-5, 12-15 in the circuit system are in the neutral position and a pressure signal from a brake-pressure-detection processor sensor 49 reaches a preset pressure; of causing a tilt-angle control unit 36 to control the tilt angle of a hydraulic motor HM; and of relatively controlling the passage resistance maintained by controlling the passage resistance control unit and the tilt angle of the hydraulic motor to maintain a brake pressure of the rotation motor.