PURPOSE: To obtain good responsiveness whether a piston rod of a servo cylinder moves in the critical direction or in the safe direction when a spool is in ordinary operation without standing at a stroke end.

CONSTITUTION: When a piston rod 14 of a servo cylinder 10 is moved in the critical direction in an ordinary operation, the oil from an oil chamber 16 of the servo cylinder 10 passes an orifice 20 and returns to an oil chamber 9 of a servo valve 1. At this time, an actuator 7 driving a spool 6 of the servo valve 1 is given an orifice opening command signal Va based on a gain Ka larger than when the piston rod 14 is moved in the safe direction, so that the spool 6 opens quickly to a prescribed angle.