Abstract: A valve including an emergency sealing device comprising a piston slidably fitted in a cylindrical bore formed at inner end of a fluid inlet passage, a seat holder slidably fitted in said piston, a main seat ring supported at the inner end of the seat holder, an auxiliary seat ring supported at the inner end of the piston, a spring arranged between said piston and said seat holder to normally hold the main seat ring against the valve element and the auxiliary seat ring away from the valve element, and a pressure medium supplying means for feeding pressure medium, when required, to press the auxiliary valve seat ring against the valve element to produce emergency sealing action and to further press the main valve seat ring against the valve element with increased pressing force.