To suppress fluctuations in the linewidth of a resist pattern caused by fine fluctuations of the thickness of an underlying film.

Sequentially deposited on a silicon substrate 1 are a silicon oxide film 2, a silicon nitride film 3 and a polysilicon film 10. A pattern of photoresist 4 is formed on the deposited polysilicon film 10 by photolithographic technique. The polysilicon film 10 and silicon nitride film 3 are subjected to a dry etching process, and then subjected to a pyro-oxidizing process to grow a silicon oxide film 2' for oxide film separation. Since there is no change on the polysilicon film 10 in the reflection factor which causes multiple interference by exposure light such as g ray (436nm), i ray (365nm), KrF (248nm) and ArF (193nm) for photolithography, there can be formed such a resist pattern that is not influenced by fluctuations in the thicknesses of the silicon oxide film 2, silicon nitride film 3 and polysilicon film 10 as its underlying layers.