To facilitate the slide motion of a movable slide body relative to a fixed slide body, and easily fabricate and machine the movable slide body by providing the combined part of the fixed slide body with a sleeve formed out of a material having high slidability and high corrosion resistance.

A valve box B at a gas side body is formed to have a cylindrical fixed slide body 7 on a water pressure reacting device A, and a slidable sleeve 8 of highly slidable acetal resin is coupled to the combined part of the fixed slide body 7. Also, the base end of a valve rod 9 is inserted in and fixed to a valve rod guide 10 as a movable slide body, and a reaction shaft 4 and the valve rod 9 are interlocked with the slidable sleeve 8 via the valve rod guide 10. In addition, the slide body 7 is formed to be capable of easily sliding on the fixed slide body 7 and frictional resistance is thereby reduced. For example, when this structure is applied to a slide position operable over a range from a water side body to a gas side body like a valve opening device to open and close a water pressure reacting valve utilizing water flow pressure, a flickering operation can be stably made at all times.