It is an object of the present invention to provide an industrially viable PSA process that requires small volumes of molecular sieve carbon and feed air. In order to achieve the above object, the PSA process uses the molecular sieve carbon satisfying the following relationships (I) and (II). (1)ln C.ltoreq.0.325+5.14.times.ln {(0.64+0.098).times.Q/(P+0.098)} (I) (2)ln U.ltoreq.2.124-0.135.times.ln C (II) where C indicates an oxygen level (ppm) in the nitrogen gas product; Q indicates a volume of nitrogen gas produced per minute per unit effective volume of a single adsorption column; P indicates an adsorption pressure (MPa); and U indicates a ratio of the volume of the feed air to the volume of the nitrogen gas produced.