PURPOSE: To simply an surely damp serge pressure in a discharge line from a pump by connecting an inlet port to the discharge line, and by connecting a spring chamber to an orifice in a pilot line branching off from the discharge line while providing a serve absorbing valve.

CONSTITUTION: An electronic controller 17 changes over a servo valve 10 so as to connect the secondary port A thereof to a pilot line 8 branching off from a discharge line 6 connected to a hydraulic cylinder 21 when a serge pressure is produced in the discharge line 6. Further, a pressure control valve 14 is changed over so as to connect its secondary port A top the secondary port A of the servo valve 10. Further, a swash plate 2 in a pump 1 is tilted toward a neutral position so as to discharge oil at a high pressure but at a low flow rate. In this arrangement, with the use of a differential pressure across an orifice 7 in the pilot line 8, the serge pressure is opened. Thereby it is possible to relieve the serge pressure from the discharge line 6 into a tank 22 even though a relief valve 13 does not open.