PURPOSE: To form high-quality gate insulating films which have uniform characteristics by eliminating unevenness in film thickness from the gate insulating films in and between elements without shaving off the surfaces of the insulating films and sufficiently cleaning the surfaces by treating the surfaces of the insulating films with pure water.

CONSTITUTION: After forming a gate oxide film 3 by thermally oxidizing a silicon substrate 1, the surface of the film 3 is cleaning by dipping 'a whole wafer in pure water which is controlled in specific resistivity to ≥17-18MΩcm. The cleaning is performed for about 20-30 minutes at a room temperature. When the surface of the film 3 is cleaned with pure water, foreign matters, such as particles, etc., are removed from the surface of the film 3 without shaving off the surface.