PURPOSE: To enhance comfort by providing a molecular sieve-type gas separation system which adsorbs nitrogen from supply air to feed product gas as oxygen- enriched gas, and controlling the system according to the actual value of oxygen content in the product gas and a required value based on control chamber pressure.

CONSTITUTION: This on-board oxygen generation system has a molecular sieve- type gas separation system 10 having a feed air supply port 11, a product gas outlet 12, and a relief outflow port 13. The gas separation system 10 is controlled by a control means 14 so that at least two sieve beds undergo an air fill/adsorption on-stream step in a circulating fashion, after which a purge/ release reproduction step is continued. In this control, the actual oxygen content sensed by a sensor 19 sensing the oxygen content of product gas is compared with the desired oxygen content of the product gas based on the control chamber pressure measured by a control chamber pressure measuring device 17, and a control signal is produced according to their deviation.