NOVELTY - A carbon-dioxide off gas containing a volatile organic compound and sulfur compound (50-10000 ppmv) and having carbon dioxide as a main component is introduced into a catalyzed-combustion device (13). The volatile organic compound and sulfur compound are oxidatively-decomposed with respect to a combustion catalyst in the catalyzed-combustion device. The combustion catalyst contains metal oxide such as zirconium oxide, titanium oxide and/or silicon oxide, and noble metal such as platinum, palladium and/or iridium.    USE - Purification of carbon-dioxide off gas discharged during manufacture of natural gas (claimed).    ADVANTAGE - The method enables purification of carbon-dioxide off gas efficiently and economically for a long period of time, without generation of nitrous oxide. The discharge amount of carbon dioxide is reduced.    DETAILED DESCRIPTION - A carbon-dioxide off gas containing a volatile organic compound and sulfur compound (50-10000 ppmv) and having carbon dioxide as a main component is introduced into a catalyzed-combustion device. The volatile organic compound and sulfur compound are oxidatively-decomposed with respect to a combustion catalyst in the catalyzed-combustion device. The combustion catalyst contains metal oxide such as zirconium oxide, titanium oxide and/or silicon oxide, and noble metal such as platinum, palladium and/or iridium. The sulfur-component concentration (except sulfur oxide) in the oxidatively-decomposed gas is 5 ppmv or less. An INDEPENDENT CLAIM is included for manufacture of natural gas, which involves supplying raw material natural gas to a slag catcher, separating raw material natural gas into a liquid phase and gaseous phase by slag catcher, separating carbon-dioxide off gas from the gaseous phase, removing moisture content in cooled and condensed source gas after separating carbon-dioxide off gas, fractionating source gas by a distilling column to remove heavy hydrocarbon and to obtain natural gas and purifying the carbon-dioxide off gas.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the apparatus for purifying carbon-dioxide off gas.    Purification apparatus (10)    Heater (11)    Preheater (12)    Catalyzed-combustion device (13)    Flow paths (14-26)