Abstract: In a method of trapping ions in a silicon oxide film or a silicon oxy-nitride film, silicon or silicon nitride as an electrode is first immersed in a nonaqueous solvent containing an electrolyte. Then, a voltage is applied between the electrode and a counter electrode so as to oxidize the surface of the silicon or silicon nitride to form a silicon oxide film or a silicon oxy-nitride film. At the same time, the ions contained in the electrolyte are caused to permeate into the silicon oxide film or the silicon oxy-nitride film and trapped therein. The method is thus capable of trapping ions in a silicon oxide film or a silicon oxy-nitride film for a short time without using any evaporation apparatus and of shifting the flat band potential of the silicon.