To excellently exhibit the decomposition/removal capacity for an alkaline gas over a long period by interposing a photocatalyst for decomposing and removing the alkaline gas in response to the irradiation with a specific light in a circulation flow passage in with respect to a device formed by interpos ing an alkaline gas absorbing filter in the midway of the circulation flow pas sage.

Relating to the air cleaner formed by interposing the alkaline gas absorbing filter 27 in the midway of the circulation flow passage for forcibly circulating room air, the photocatalyst 40 for decomposing and removing the alkaline gas in response to the irradiation with ultraviolet ray is carried on the alkaline gas absorbing filter 27. That is, the alkaline gas absorbing filter 27 is constituted of plural filter elements 27A-27C, on each of which the photocatalyst 40 is carried, and an ultraviolet lamps 42 for irradiating the photocatalyst 40 with ultraviolet ray are arranged among the filter elements 27A-27C. And the photocatalyst is arranged on the inner wall surface of a gas circulating duct or a gas feed duct constituting the circulation flow passage to reduce the load of the alkaline gas absorbing filter 27.