A method for oxidation of an object to be processed is provided wherein an oxide film can provide favorable film quality and a laminate structure of nitride film and oxide film can be obtained by a thermal oxidation of a nitride film.In a method for oxidation of a surface of an object to be processed in a single processing container 8 which can contain a plurality of objects to be processed, at least a nitride film is exposed on said surface, and said oxidation is performed by mainly using active hydroxyl/oxygen species in a vacuum atmosphere, setting a processing pressure to 133 Pa or below, and setting a processing temperature to 400.degree. C. or above. Under these conditions, high interplanar uniformity is maintained and oxide films with favorable film quality are obtained by oxidizing nitride films on the surfaces of a plurality of objects to be processed.