Abstract: In a method of manufacturing a semiconductor device, after forming a poly silicon film 54 on a surface of a silicon substrate 51, a silicon nitride film 55 is formed in accordance with a desired pattern and a local oxidation process is carried out to form a field oxide film 56 having a large thickness. Then, after removing the silicon nitride film 55, the poly silicon film 54 is fully converted in to a silicon oxide film 58 and then the thus converted silicon oxide film is removed by wet etching to expose a clean surface of the silicon substrate 51. The poly silicon film does not constitute an oxygen source, so that during the local oxidation, a lateral diffusion of oxygen is prevented and a generation of bird's beak can be suppressed. Further, the poly silicon film serves as a buffer, no stress remains in the surface of the silicon substrate.