PURPOSE: To make it possible to continue the drive of a operated body by parallelly connecting the operating portion of a hydraulic cylinder to the operated body, providing a pressure compensator in a return pipe and providing an inoperative trouble detecting device at a servo valve.

CONSTITUTION: Operating portions 4a, 4b consisting of double acting hydraulic cylinders 6a, 6b servo valves 7a, 7b and bypass valves 9a, 9b are parallelly connected to an operated body 1 and a pressure compensator for rising the pressure of return operating oil is provided in a return pipe. Servo valve inoperative trouble detectors 35, 36, 37 operated by the excessive operating force of the servo valve 7a are provided between the spool 12, the link 13 of the servo valve 7a and an arm 34. Accordingly, when the servo valve 7a is out of order, a trouble is thereby detected in a moment through an operator's counter pressure operation and it is possible to be switched to the other servo valve 7b so that the drive of the operated body can be continued.