PURPOSE: To contrive the improvement of a dielectric breakdown strength without a significant increase in the number of processes by a method wherein, after silicon is made to contain in a nitride film, an oxidation treatment is executed.

CONSTITUTION: Silicon is made to contain in a nitride film 5 for forming an insulating layer on a floating gate electrode 3 and thereafter, the film 5 is oxidized. A process to make silicon contain in the film 5 can be performed by an ion implantation method, wherein silicon is used as a dopant, for example, By making silicon contain in the film 5 in such a way, the amount of silicon in the nitride film is augmented and in case an oxidation treatment is performed, the oxidizing rate is improved. Accordingly, even though the film 5 is oxidized simultaneously with other gate oxidizing process, an Si oxide film 6 can be formed thickly on the film 5. Thereby, the improvement of a dielectric breakdown strength is contrived.