To improve the turning operability in a small-turning radius posture in a turning work machine.

Crossover relief valves 11, 12 provided between two motor lines 8, 9 feeding/discharging hydraulic oil to a turning hydraulic motor 10 are provided with spring adjusting cylinders 15, 16 adjusting the exciting forces of relief springs 13, 14. When the small-turning radius posture of a front work device is detected by a detecting means 28, a solenoid selector valve 25 is switched, pilot lines 24, 26 for extracting the turning operation pilot pressure are communicated with each other, and a pressure reducing valve 18 is controlled by the turning operation pilot pressure speed-controlling the turning hydraulic motor 10. The pressure reducing valve 18 reduces the pilot pressure for setting the relief set pressure outputted to spring adjusting cylinders 15, 16 in response to an increase of the turning operation pilot pressure to reduce the set pressure of the crossover relief valves 11, 12.