After cleaning a surface of a silicon substrate (1), impurities and natural oxide film existing on the silicon substrate (1) are removed by soaking the silicon substrate (1) in a 0.5%-by-volume HF aqueous solution for 5 minutes. The silicon substrate (1) is rinsed (cleaned) with ultrapure water for five minutes. Then, the silicon substrate (1) is soaked for 30 minutes in azeotropic nitric acid heated to an azeotropic temperature of 120.7.degree. C. In this way, an extremely thin chemical oxide film (5) is formed on the surface of the silicon substrate (1). Subsequently, a metal film (6) (aluminum-silicon alloy film) is deposited, followed by heating in a hydrogen-containing gas at 200.degree. C. for 20 minutes. Through the heat processing in the hydrogen-containing gas, hydrogen reacts with interface states and defect states in the chemical oxide film (5), causing disappearance of the interface states and defect states. As a result, the quality of the film can be improved. Thus, it is possible to form a high quality (with low leak current density), extremely thin silicon dioxide film on the silicon substrate at a low temperature with excellent film-thickness controllability.