Selective adsorbent for nitrogen and method for separating air using the same

+++A nitrogen selective adsorbent comprises a zeolite of a faujasite crystalline structure containing Li and at least one of NH and H as essential cations, and has a nitrogen adsorption characteristic represented by specific correlation between the number of associated Li ions per unit lattice of a zeolite crystal and the amount of adsorbed nitrogen per unit lattice of the zeolite crystal. An air separation method employs the aforesaid nitrogen selective adsorbent for separation between nitrogen and oxygen by selective adsorption of nitrogen in air.