A method of post-deposition treatment for silicon oxide film includes: providing in a reaction space a substrate having a recess pattern on which a silicon oxide film is deposited; supplying a reforming gas for reforming the silicon oxide film to the reaction space in the absence of a film-forming precursor, said reforming gas being composed primarily of He and/or H2; and irradiating the reforming gas with microwaves in the reaction space having a pressure of 200 Pa or less to generate a direct microwave plasma to which the substrate is exposed, thereby reforming the silicon oxide film.