PURPOSE: To form an SOI thin film in a uniform thickness by a method wherein a semiconductor film capable of sufficiently absorbing a laser beam is formed on a semiconductor oxide film having regularly protruding and recessed parts, the film is then irradiated with the laser beam and heated, an extremely thin film is heated, melted and recrystallized and, after that, all upper-layer films are removed.

CONSTITUTION: A film of a semiconductor oxide film is processed to be a stripe pattern; after that, a silicon oxide film is formed on it; a silicon oxide film 6 having protruding and recessed parts is formed. The surface of a substrate where these films have been constituted is irradiated with an argon gas laser and the laser is scanned; a recrystallization process progresses in a silicon oxide film 3 from a low-temperature part; a single-crystal silicon thin film is formed under thick parts of the polysilicon oxide film 6. The polysilicon oxide thin film 3 is recrystallized; after that, a silicon film 7 is dried and removed. Then, the polysilicon oxide film 6 is removed. Then, a silicon nitride film 5 is removed; lastly, a silicon oxide film 4 is removed. Thereby, an SOI substrate having a single-crystal silicon thin film of about 0.1μm or lower can be formed on a silicon oxide film 2 with better accuracy in a film thickness.