NOVELTY - A carbon-monoxide-modifying apparatus (50b) consists of a carbon-monoxide modification unit (10) having a catalyst layer (5) consisting of a modification catalyst, for modifying to-be-processed gas (G0) flowing inside, and a carbon-dioxide removal unit (51). The catalyst layer reduces carbon monoxide conversion ratio as the concentration of carbon dioxide in gas flowing through increases. The concentration of carbon dioxide in to-be-processed gas is lowered in the carbon-dioxide removal unit, and to-be-processed gas is supplied to the modification unit.    USE - Apparatus is used for modifying carbon monoxide (claimed) in fuel cells.    ADVANTAGE - The apparatus improves the carbon monoxide conversion ratio without increasing the usage amount of modification catalyst. Reduction in efficiency of carbon monoxide modification by poisoning of modification catalyst is prevented.    DETAILED DESCRIPTION - A carbon-monoxide-modifying apparatus produces carbon dioxide and hydrogen by reacting carbon monoxide and water contained in to-be-processed gas. The apparatus consists of a carbon-monoxide modification unit having a catalyst layer consisting of a carbon-monoxide-modification catalyst, for modifying gas flowing inside, and a carbon-dioxide removal unit for removing carbon dioxide contained in the introduced gas and sending out process gas whose carbon dioxide concentration is lower than the introduced-gas downstream. The catalyst layer reduces carbon monoxide conversion ratio as the concentration of carbon dioxide contained in the gas flowing through increases. The concentration of carbon dioxide contained in to-be-processed gas is lowered in the carbon-dioxide removal unit, and to-be-processed gas is supplied to the modification unit. An INDEPENDENT CLAIM is included for modification of carbon monoxide.    DESCRIPTION OF DRAWING(S) - The drawing shows a conceptual view of the carbon-monoxide modification apparatus.    Catalyst layer (5)    Carbon-monoxide modification unit (10)    Carbon-monoxide-modifying apparatus (50b)    Carbon-dioxide removal unit (51)    To-be-processed gas (G1)