PURPOSE: To keep a low oxygen concn. atmosphere in a chamber by providing an oxygen-enriching apparatus utilizing a high polymer gas separation membrane and a means for replenishing atmospheric air into the chamber by the amt. corresponding to that of oxygen-enriched air discharged to the outside of the chamber.

CONSTITUTION: The air in a storage chamber 1 is supplied to a high polymer gas separation membrane module 4 by a feed fan 2 through a duct 3. The oxygen-nitrogen separation is carried out in the module 4 by operating a vacuum pump 5, and the chamber air is separated into oxygen-enriched air and nitrogen- enriched air. The oxygen-enriched air in discharged to the outside of the chamber through a discharge line 6, and the nitrogen-enriched air is returned to the inside of the storage chamber 1 through a recovery line 7. The amt. of air corresponding to that released from the storage chamber 1 is always replenished by a fan 8 for replenishing atmospheric air. In addition, the oxygen concn. in the storage chamber 1 is measured by an oxygen densitometer 9.