PURPOSE:To prevent the deterioration of gate withstand voltage by uniformly forming a gate insulation film and eliminate the possibility of disconnections of a wiring metal at the stepwise difference in a flat structure by covering an active element region with a selective oxide film. CONSTITUTION:A polycrystalline Si 302 is deposited on an insulation substrate 301, and then an oxide film 303 is formed by thermal oxidation. Next, after forming and processing the polycrystalline Si serving as a gate electrode 304, an oxide film 305 is formed by thermal oxidation. Then, an Si nitride film 306 is formed at the active element region, and processed, element isolation is performed by selectively oxidizing the polycrystalline 302 with the film as a mask, and then the Si nitride film 306 is removed. Thereafter, the source and drain regions 308 are formed by ion-implanting boron, etc. Finally, a contact hole for leading out the wiring is formed after depositing an NSG film serving as an interlayer insulation film 309, or a PSG film over the entire surface, and the wiring metal 310 such as Al is wiring-formed.