PURPOSE: To effectively treat organic halogen compounds at low temperatures by installing a catalytic decomposition device packed with an organic halogen compound decomposition catalyst having a strong acidic point and a washing column for removing gaseous hydrogen halide generated by the decomposition of the organic halogen compounds.

CONSTITUTION: In a device where waste gas contg. organic halogen compounds (e.g. carbon tetrachloride) is treated and purified, a catalytic decomposition device 4 packed with an organic halogen compound decomposition catalyst having a strong scidic point (e.g. zeolite) and a washing column 6 for removing gaseous halogen halide generated by the decomposition of the organic halogen compounds are installed. And in the preceding stage of the catalytic decomposition device 4 are provided with a concentrator 1 and gas heaters 2, 3. Further, in the poststage of the catalytic decomposition device 4 or the washing column 6, a carbon monoxide oxidizing device is provided, and in the prestage of the catalytic decomposition device 4, a steam feed line 7 is provided. And the organic halogen compound decomposition catalyst uses oxides of one or two or more of Si, Al, Ti, Zr, etc.