PURPOSE: To prevent impurities being included in an oxide film from diffusing in a film consisting of polycrystalline silicon by a method wherein the film consisting of polycrystalline silicon is formed on the surface of the oxide film through an undoped oxide film.

CONSTITUTION: A base 2, an emitter 3 and so forth are formed by diffusing impurities using an oxide film 4 as a mask, and after that, an oxide film 9 is formed on the forming position of a polycrystalline silicon film in advance of the formation of the polycrystalline silicon film. As this oxide film 9 has not been used as a mask at the diffusion time of impurities like the oxide film 4, impurities have not been doped on the oxide film 9 at all. A film 7 consisting of polycrystalline silicon is formed on the surface of the oxide film 9. According to such a constitution, when the film 7 is formed by a vapor-phase growth method, the oxide film 9 is heated, but there is no possibility that impurities are diffused in the film 7 from the oxide film 9 even though the oxide film 9 is heated, because the oxide film 9 is not including impurities. Even supposing there is a possibility that impurities exhale from the oxide film 4, the exhalation is shieled by the oxide film 9 and impurities do not reach up to the film 7. As a result, it is surely prevented for impurities to diffuse in the film 7.