PURPOSE: To shear biting of dust or the like by a holding force of a lock mechanism and improve the reliability by holding a play motion part of a floating link by a lock mechanism movable in the motion direction of a spool of a control valve to keep it at a predetermined position when a trouble signal is inputted.

CONSTITUTION: Displacement of a floating link 3 connected with a spool 2a of a control valve 2 is linked with that of an output end la of an actuator 1 through a link mechanism 4. In the meanwhile, a lock mechanism 5 is composed of a cylinder 5a, a piston 5b, an oil chamber 5m, a spring 5c, a fork 5e, a roller 5g, a bracket 5h, a pin 5i, and a receiver 5j, and it is actuated by an oil pressure through a solenoid valve 9 at the time of a trouble. A cam 3a of a floating link 3 is held and fixed by grips 5l, 5k. Dust or the like bitten at the spool 2a, if any, is therefore sheared by a holding force of the lock mechanism 5, the reliability is improved, and the size can be compact.