PURPOSE: To readily and rapidly remove impurities from nitrogen gas by cooling the nitrogen gas containing oxygen as an impurity with liquid nitrogen to remove most of the impurities and further selectively adsorption-removing oxygen gas from the nitrogen with molecular sieves.

CONSTITUTION: Nitrogen gas (purity: 99.9-99.99%, impurities: O2, H2O, CO2, CH4, etc.,) obtained by a pressure swing adsorption method (PSA), etc., is sent from a line 51 into low temperature adsorption towers 6a and 6b cooled with N2 gas supplied through a liquid nitrogen storage tank 8 and a vaporizer 9. An active carbon adsorbent 60 and molecular sieves absorbent 61 such as NaA type synthetic zeolite selectively adsorbing only O2 are filled on the upstream sides of the adsorption towers 6a and 6b and on the downstream sides, respectively, and H2O, CO2 and CH4 are adsorption-removed with the adsorbent 60. The O2-removed N2 gas and the N2 gas with which the adsorption towers 6a and 6b are cooled are combined into a line 52 through valves 63 and 64 and a valve 66, respectively, and the thus obtained high purity N2 having a purity of ≥99.999vol.% is stored in a product N2 storage tank 7.