A process for separating nitrogen gas higher in purity than conventional products from a gas mixture predominantly composed of nitrogen gas and oxygen gas by the pressure swing adsorption system using carbon molecular sieve as adsorbent is characterized by the manner of communicating an adsorber after adsorption step with an adsorber after regeneration step for gas transfer from the former adsorber to the latter, the operation of discontinuing the gas transfer before establishment of pressure equalization, and the regeneration promotion by causing back flow of part of product gas.The process can separate 99.99% pure nitrogen gas, which is never obtainable in the prior pressure swing adsorption processes, from air as raw material in an economic manner.