A semiconductor device fabrication method is disclosed. The method comprises an insulating film forming step of forming an insulating film on a semiconductor substrate; a trench forming step of forming a trench for device isolation in a predetermined part of the semiconductor substrate; a trench filling step of forming a buried oxide film filling the trench; a polishing step of polishing the buried oxide film on the semiconductor substrate until the insulating film is exposed; a thickness measuring step of measuring the thickness of the insulating film remaining after the polishing; an etching amount determining step of determining an etching amount of etching the polished buried oxide film based on the measured thickness of the remaining insulating film; and a buried oxide film etching step of etching the polished buried oxide film based on the determined etching amount.