PURPOSE: To return an actuator to its neutral position by connecting a floating link to the spool of a control valve and holding the idling portion of the floating link when a trouble signal is input.

CONSTITUTION: A floating link 23 is supported pivotally on the spool 22a of a control valve 22 and the floating link 23 is connected to the spool 21a of an actuator 21 via a link mechanism 24, and then a lock cylinder 25 is arranged oppositely to the end portion 23a of the floating link 23. In this manner, the idling portion of the floating link 23 can be held when a trouble signal is input, so that the actuator 21 can be returned to its neutral position in such a way that the position of the actuator 24 is fed back to the spool 22a of the control valve 22 via the link mechanism 24 and the floating link 23 at time of trouble.