To avoid the stop of the whole control system of a servo valve by outputting the command signal to a spool driving means through a separately provided output means when an abnormality is present in a displacement sensor or a feedback mechanism.

In a normal condition, the command signal from a main controller 8 is received by a current PWM drive circuit 14 through a comparator 17 of a first output means 12 and a first amplifier 18. The command signal is compared with the displacement signal from a displacement sensor to detect the movement of a working piston 4 by the comparator 17, and the current flowing in an armature coil 5 is controlled so as to eliminate the difference. When the displacement sensor is failed, the failure is diagnosed by an abnormality diagnosis part 16, first and second change-over switches 10, 11 are automatically switched, and connected to a second output means 13 side. The command signal from the controller 8 is received by the current PWM drive circuit 14 through a second amplifier 19 of the second output means 13, and the current flows in the armature coil 5.