PURPOSE: To flatten the surface of a flattening material without giving damage to a desired film thickness, by heaping the flattening material on a semiconductor thin film with its surface recessed/projected on an insulating layer formed on a substrate and next by performing grinding and mirror-polishing processes.

CONSTITUTION: After an SiO2 film 2 is formed on an Si substrate 1, the film 2 is etched in a prescribed pattern. Next, a single crystal Si film 3 is formed on the SiO2 film 2. The surface of this single crystal Si film 3 is recessed/ projected as a result of its epitaxial growth via the SiO2 film. A flattening material, for example, polycrystalline Si 4 is heaped on this single crystal Si film 3 with its surface recessed/projected. The polycrystalline Si 4 is ground to flatten the surface by a grinding process. This grinding process is performed so deep that no damage is given to the single crystal Si film 3 of a desired thickness.