PURPOSE: To enable fiber to be prevented from its discoloring and fading by subjecting air in a storage space for fiber, etc., to oxidation treatment and then absorption treatment and removing nitrogen dioxide contained in the aforementioned air.

CONSTITUTION: Air is sucked and passed through an oxidation bottle containing a potassium permanganate solution acidic with sulfuric acid in a storage space for fiber such as a storehouse to oxidize nitrogen monoxide in the air into nitrogen dioxide. The resultant air is then passed through an absorbing solution composed of a mixed solution of naphthylethylenediamine dihydrochloride, sulfamic acid and glacial acetic acid to remove the nitrogen dioxide caused by exhaust gases from heating equipment or automobiles contained in the air. Thereby, the obtained air is subsequently fed to the aforementioned storage space.