PURPOSE: To improve finish accuracy in active width, by electrically charging a pad oxide film causing a break-down at a weak point of the pad oxide film by the electric field, and forming a nitride film after a self-healing phenomenon thereof.

CONSTITUTION: When a pad oxide film 2 is formed on a P-type silicon substrate 1, a weak spot 3, at which nitrogen and water vapor are easily diffused, is formed in the oxide film 2. In this case, the surface of the oxide film 2 is charged with a negative charge 4. When an electric field caused by the negative charge 4 becomes stronger than the dielectric strength at the weak spot 3, break-down takes place and a current is carried, and Joule heat is generated. Elements, such as silicon and oxygen, are put into a flow state so that a stable SiO2 bondage is generated. When these steps are repeated, the weak spot 3 is diminished and finally extinguished so that the pad oxide film without the weak spot can be obtained. An oxidation-proof nitride film 5 is formed thereon. Reaction of SiON in an active region can be suppressed during a field oxidation step and variation in active width can be reduced.