Between a driving base operated by an air cylinder and a lever member fixed to a valve shaft integral with a valve plate, a connecting mechanism for connecting the driving base and a lever member, capable of displacement, and a cam mechanism for tilting the lever member are interposed, and a stopper mechanism for stopping the lever member at an opposing position of the valve plate or a full-open position, and after the lever member is stopped by the stopper mechanism at the opposing position or the full-open position, the lever member is tilted by the cam mechanism so as to incline the valve plate and to press a valve seal to a valve seat portion or a wall face in the periphery of an opening.