The present invention relates to an apparatus to form a thin film on a specimen in a chamber by introducing electromagnetic waves into the chamber through a dielectric provided on a window of the chamber and activating the material gas in the chamber, and the forming apparatus according to the present invention has such a construction that an electrode connected to a high-frequency generating source is mounted near the dielectric. Consequently, when a high frequency is applied to the electrode, self-biases are generated on the inner surface of the dielectric (the surface on the side of the specimen), the plus ions in the chamber being attracted to the inner surface to strike thereon, and the sputtering being caused on the inner surface by the incidence energy. As a result, due to the sputtering effect, the generation of a pile of films on the inner surface of the dielectric can be prevented, while in the case where films are already piled on the inner surface of the dielectric these films can be removed.