To ensure detection of the inflated and shrunk condition of an inflated sheet and of the position of the inflated sheet with a mechanical means.

An inflated sheet 6 formed of elastic material, inflated or shrunk ranging between an inflation position of pressing it against a valve seat 5 and a shrinkage position of separating it therefrom is provided on a base material 7, a detecting bar 22 set in and out in association with the inflating and shrinking operation of the inflated seat 6 is provided so as to pass a valve box 2 and the base material 7 from the outside through a pressure fluid sealing space S and an inflation detecting limit switch 27 and a shrinkage detecting limit switch 28 are provided on a supporting member 24 for detecting the backwarding position of the detecting bar 22 in the shrinkage of the inflated sheet 6 and the forwarding position of the detecting bar 22 in the inflation, respectively.