PURPOSE: To obtain MOSFET of high performance by introducing plasma anodization and a metal layer having high conductivity, and thereby enabling the formation of a thinner gate insulation film at low temperature.

CONSTITUTION: Al source and drain wirings 7 and 8 are formed selectively, containing a part of an insular poly-Si 2 on a glass plate 1, and one of these wirings is extended to the periphery of the substrate 1 via a scribed grid and connected to a metal electrode 15. A bias positive to an O2 plasma generating region is given to the electrode 15 to conduct plasma anodization, and SiO2 is formed on the poly-Si 2 which is exposed, while Al2O3 17 having almost the same thickness as SiO2 16 is formed on the wirings 7 and 8. Openings are made in the film 17, Al wirings 7' and 8' are given thereto, while a gate electrode 6 is also given onto a gate oxide film 16, containing a part of the film 17, and thus the device is completed. According to this constitution, the gate oxide film is excellent in quality and free from pinholes even when the thickness thereof is about 500, the increase in gm is obtained thereby, the whole process can be executed at a low temperature of 300°C or below, the shortening of the process and the saving of electric power can be attained, and thus the cost of the device is reduced.