There is provided a method of forming a silicon oxide film on a target surface on which a silicon oxide film and a silicon nitride film are exposed. The method comprises placing a workpiece having the target surface on which the silicon oxide film and the silicon nitride film are exposed, in a processing container under a depressurized atmosphere; forming a spacer silicon nitride film to be a sacrificial film on the target surface on which the silicon oxide film and the silicon nitride film are exposed; and substituting the spacer silicon nitride film with a substitution silicon oxide film by supplying thermal energy, oxygen radicals and hydrogen radicals onto the workpiece.