A large amount of a medium containing a volatile chlorinated organic compound (VOC) with a low concentration is passed through an adsorber filled with an adsorbent which adsorb and desorb the VOC to collect the VOC in the medium in the adsorber; a gas containing water vapor is then brought into contact with the adsorbent to desorb and discharge the VOC from the adsorber; the desorbed gas is cooled to condense the VOC and water; a small amount of the VOC with a high concentration vaporized from the condensate is further irradiated with light in the presence of chlorine to decompose the VOC; and the decomposed product is subsequently treated. This constitution can allow a large amount of the VOC with a low concentration to be irradiated with light in the presence of chlorine to efficiently treat the VOC.