PURPOSE: To prevent gate oxide film formed after forming field oxide film from being thinned by forming an oxide film by oxidizing a specific region on the substrate surface, applying ultraviolet rays while allowing a solution containing fluorine to contact the substrate surface, and etching the oxide film and the substrate with nearly equal rates.

CONSTITUTION: Field oxide films 14 are formed at the left and right of Si substrate 1 and a thin pad oxide film 12 for connecting them is formed. Then, the surface part of the field oxide films 14 and the pad oxide film 12 are removed. Then, the field oxide films 14 are placed in oxygen atmosphere, ultraviolet rays are applied to them, at the same time 0.05%HF is applied over the substrate 1, thus completely etching the field oxide film 14 and the pad oxide film 12 and hence etching the Si substrate 1 as an exposed ground at nearly an equal rate as that of the field oxide films 14 due to the synergistic effect of 0.05%HF and ultraviolet rays and hence preventing thinning of a gate oxide film.