PURPOSE: To improve the yield and the reliability of an MISFET device by injecting nitrogen ions through an oxidized film on an element forming region isolated with selectively oxidized layer, and thermally nitriding it after removing the oxidized film in nitriding atmosphere to form a gate film.

CONSTITUTION: An oxidized film 4(200∼2,000) is formed on an Si substrate formed thickly with a selectively oxidized film 2, and nitrogen ions are injected in the amount of 1015∼1017cm-2 on the overall surface, thereby forming an injected layer 5. Then, the film 4 is removed. NH3 or N2 or mixture gas of NH3 or N2 is used as an atmosphere, it is heat treated at 1,000∼1,200°C, and the layer 5 is converted to thermally nitrided film 6. Subsequently, a polysilicon gate electrode 7 is formed, source and drain diffused layers 8, 9 are formed, and an FET having the film 6 as a gate film is prepared. In this manner, the film 6 is increased in thickness (to approx. 2,000) readily, and is adapted for the MISFET, thereby increasing the yeild and the reliability.