PURPOSE: To produce high-purity gaseous nitrogen by using fuel more than the theoretical combustion amount of oxygen by prescribed amount in gaseous nitrogen containing oxygen and removing moisture, gaseous carbon dioxide and unburned fuel by a pressure swing adsorption method.

CONSTITUTION: Gaseous nitrogen containing oxygen (e.g. off-gas of a chemical plant) is supplied via a pipeline 1 and heat-exchanged with catalytic combustion exhaust gas in a heat exchanger 2 and thereafter led to a catalytic combustor (oxidative catalyst) heated at ≥100°C via a pipeline 3. On the other side, fuel (e.g. ethylene) more than the theoretical amount of oxygen by 1-20wt.% in gaseous nitrogen containing oxygen is supplied from a pipeline 4. Herein gaseous oxygen in gaseous nitrogen containing oxygen is completely consumed. Water and gaseous carbon dioxide are produced. Combustion exhaust gas discharged from the catalytic combustor gives heat of combustion to gaseous nitrogen containing oxygen in the heat exchanger 2 via a pipeline 5 and is cooled and supplied to a pressure swing adsorption device (an adsorbent is alumina, activated carbon and zeolite). Moisture CO2 and unburned gaseous fuel are removed and high-purity gaseous nitrogen is discharged from a duct 7.