The whole areas of both surfaces (10a and 10b) of a silicon wafer (10) are covered by silicon nitride films (13, 14) respectively through the intermediary of pad oxide films (11 and 12), and the pad oxide film (11) and the silicon nitride film (13) on the front surface (10a) of the wafer are patterned in desired regions and therefore the front surface (10a) is partially exposed. On the other hand, the pad oxide film (12) and the silicon nitride film (14) on the reverse surface (10b) of the wafer are removed, so the whole area of the reverse surface (10b) is exposed. By simultaneously oxidizing the regions exposed partially on the front surface (10a) of the wafer and the whole area of the reverse surface (10b) of the wafer, silicon dioxide films (15 and 16) are grown on those areas of the wafer.