An ion generating device generates O2−(H2O)n (where n is a natural number) as negative ions and H+(H2O)m (where m is a natural number) as positive ions, and discharges those ions into the air so that airborne germs are killed through an oxidation reaction by hydrogen peroxide H2O2 or radical hydroxyl OH generated through as an active species a chemical reaction between the negative and positive ions. Satisfactory sterilization is achieved when the negative and positive ions are generated in such a way that the concentrations of the negative and positive ions are both 10,000 ion/cc at a distance of 10 cm from the point at which they are generated.