PURPOSE: To prevent accumulated films from entering a reaction vessel even if the accumulated films formed at the inwall of a pressure vessel exfoliate by storing a reaction tube the upper end of which is blocked and the upper end of the side wall of which is provided with, at least, one opening.

CONSTITUTION: The upper end of a reaction tube 2 is blocked with a cover 28 which has a diameter larger than the diameter of the upper end of this reaction tube 2. At the upper end of the sidewall of the reaction tube 2 are provided plural pieces of openings 32a, 32b, 32c, 32d, 32e, and 32f. A pressure- resistant pressure vessel 6 surrounds the reaction tube 2 so that space 4 may be formed between itself and the reaction tube 2. The introduced material gas decomposes, being heated by a heater 20, and forms a film such as polycrystalline silicon film, a silicon nitride film, etc., according to the kind of material gas. Even if the film formed at the inner face of the sidewall of the pressure vessel 6 comes off, since the openings 32a-36f are shielded by a downward projection 30, the separated pieces never enter the reaction tube 2.