PURPOSE: To efficiently concentrate the isotopes of nitrogen and oxygen by joining a concentrating process of the isotopes of nitrogen and oxygen dependent on the low-temp. distillation method of nitrogen monoxide to a concentrating process of the isotope of nitrogen dependent on an NO-HNO3 exchange method.

CONSTITUTION: When regulated nitric acid is supplied at constant flow rate to an NO-HNO3 exchange tower 10 from the top thereof, nitric acid reached the bottom of the tower is supplied to a reflux tower. It is allowed to react with nitrous gas and changed into nitrogen monoxide and sulfuric acid. Further in an exchange tower 10, the exchange reaction of the isotope of nitrogen is caused by nitrogen monoxide and nitric acid and 15N is concentrated in nitric acid. On the other hand, high-grade nitrogen oxide is contained in gaseous nitrogen monoxide discharged from the top of the tower. High-purity gaseous nitrogen monoxide is obtained in an NO purifier 20. Thereafter 15N, 17O and 18O are concentrated in an NO distillation tower and 15N, 18O are recovered in the bottom of the tower. 170 is recovered in the intermediate part thereof and 14N, 16O are recovered in the top of the tower. In such a way, the isotopes of nitrogen and oxygen can be efficiently concentrated.