PURPOSE: To impart a self-cleaning function by arranging in a bore a wire end member having a working surface for a pressure medium and capable of being partially pushed out of the bore in such a way that the member is returned into the hole under the action of a returning force.

CONSTITUTION: A bore 13 between bores 12, 14 is formed in a small diameter and set to an axial length that roughly corresponding to the diameter. A conical bore segment 16 having a cone angle of about 90° is provided between the bores 13, 14 as a moving part. A coil spring 19 is stored in the bore 14 and a hydraulic unit 15 is supported at the downstream end of the spring 19. A wire end segment 20 extending in the longitudinal direction of the bore 13 is formed at the upstream end of the spring 19. The end face of the wire end segment 20 is formed into a perpendicular end face 21. A throttle is located to resist a pressure medium flowing through a stock indicated by the arrow II, and if a great fluid pressure works on the end face 21 because of clogging of the throttle, etc., the wire end segment 20 reaches the area of the bore segment 16 and opens the cross section of the bore 13 to undo the clogging. Next, the throttle is restored to its original position by the spring 19.