A gas inlet is disposed in a lower portion of a reaction chamber, a copper substrate is disposed in an upper portion thereof, and a tungsten catalytic body heated to 1600° C. is disposed midway between the two. Ammonia gas introduced from the gas inlet is decomposed by the tungsten catalytic body, a chemical species generated by the decomposition reacts with a surface of the copper substrate, and reduces and removes a contaminant on the copper surface, and a Cu3N thin film is formed on the copper substrate surface. This Cu3N film has the action of a film which prevents the oxidation of copper. This Cu3N film is thermally decomposed and removed when heated to temperatures of not less than 300° C., leaving a clean copper surface behind.