Disclosed is a gas valve capable of switching gases to be introduced within a vacuum chamber with high speed thereby enhancing the controllability of the composion of a semiconducting thin film growing on a substrate and shortening the time required for growth of the thin film. The gas valve comprises a bendable film between a pair of parallel plate electrodes whereby operating the film by an electrostatic force and opening and closing a port for releasing gas to a substrate mounted on the wall surface of a gas chamber and a port for exhausting an unnecessary gas to an exhaust passage. The gas valve is mounted in the vicinity of the substrate within the vacuum chamber for supplying a working gas in a minimum amount required for the film growth.