PURPOSE: To form an isolation insulating film in which no step and no bird beak occur by increasing its thickness by forming a groove on an isolation film forming region, burying a narrow groove for forming the insulating film with an SOG film, burying a wide groove with a CVDSiO2 film and polishing it.

CONSTITUTION: An Si3N4 film 2 is grown as an anti-polishing film on an Si substrate 1, and a groove is formed on a field region. The substrate 1 is rotatably coated with a spin-on-glass(SOG) film 3 in a thickness for closely burying in a pattern. Then, an SiO2 film 4 is deposited in a thickness on the field region to become the same level as that of the film 2. With the film 2 as a stopper the film 4 is polished, the film 2 is removed, and elements are formed in the exposed substrate. Thus, a surface of the substrate is flattened, and since an insulating film buried in the field region is not thermally oxidized, no bird's beak is generated.