PURPOSE:To microminiaturize a semiconductor device and enhance the integrity of the device by selectively oxidizing an electrode material film on the entire surface of a substrate to form predetermined electrode pattern and forming an impurity layer of a region adjacent to the electrode pattern by an ion implantation process. CONSTITUTION:A field oxide film 2 is formed on the surface of a p-type silicon substrate 1 by a selective oxidation process or the like. A gate oxide film 3 is formed on the surface of the substrate, and a polycrystalline silicon film 4 is subsequently formed thereon. A silicon oxide film 5 is then formed thereon, photoetched to selectively retain the film 5 on the gate electrode forming region, and with the film 5 as a mask to form n<+>-type source impurity layer 6 and drain impurity layer 7 by ion implantation process. With the film 5 as an oxidation resistance mask the layers are thermally oxidized to convert the portion except the gate electode region of the film 4 into a silicon oxide film 42 and to form a gate electrode 41 under the film 5. Contact holes are formed in the source and drain electrode leading regions, respectively, and electrodes are then arranged.