PROBLEM TO BE SOLVED: To easily change the drive/brake pressure of a hydraulic swing motor while leaving the structure of a shock-less function as it is, and to change the maximum drive torque and brake torque of the hydraulic swing motor.SOLUTION: In a swing motor unit 4, there are arranged first swing relief valves 44a, 44b having the shock-less functions which limit the drive/brake pressure of the hydraulic swing motor 40 so as not to exceed first set pressure, and second swing relief valves 46a, 46b which limit the drive/brake pressure of the hydraulic swing motor so as not to exceed second set pressure. By switching valves 47a, 47b by operating a relief-pressure change command switch 8, there is selected one of a mode at which the relief characteristic of the first swing relief valve is made effective as it is, and a mode at which there can be obtained a relief characteristic at which the first set pressure of the relief characteristic of the first switch relief valve is lowered to the second set pressure of the second swing relief valve.