Uniform oxynitride and nitride films can be formed by low-temperature and high-speed nitriding reaction not dependent on the nitriding time or nitriding temperature. A solid dielectric is provided on at least one of opposed surfaces of a pair of electrodes opposed to each other under a pressure of 300 (Torr) or higher, a nitrogen gas containing an oxide equal to or lower than 0.2% is introduced into a space between the pair of opposed electrodes, an electric field is applied to the nitrogen gas, and the resulting N.sub.2 (2.sup.nd p.s.) or N.sub.2 (H.I.R) active species is brought into contact with an object to be processed to form an oxynitride film/nitride film on a surface of the object to be processed.