NOVELTY - Modification unit comprises: (a) modifier which heats the source gas containing a hydrocarbon raw material and water vapor with a combustor in a modification catalyst; (b) carbon monoxide (CO)-modifying device; (c) CO selective-oxidation device; (d) methanation device to which the methanation of the CO is carried out; (e) water-flow path where the water utilized for the water vapor circulates; (f) combustion gas flow path; and (g) waste gas cooler.    USE - The modification unit is useful for manufacturing fuel cell system (claimed).    ADVANTAGE - The heat of combustion gas can be recovered very favorable and the energy efficiency of the modification unit can be improved.    DETAILED DESCRIPTION - Modification unit comprises: (a) modifier which heats the source gas containing a hydrocarbon raw material and water vapor with a combustor in a modification catalyst, and generates the reformed gas containing hydrogen gas (H2); (b) carbon monoxide (CO)-modifying device which the reformed gas generated with this modifier is supplied, and transforms CO in the reformed gas to a carbon dioxide (CO2) according to a CO-modifying catalyst; (c) CO selective-oxidation device which the reformed gas processed by this CO modifying device is supplied, and oxidizes CO which remains in the reformed gas to CO2 according to CO selective-oxidation catalyst; (d) methanation device to which the methanation of the CO is carried out; (e) water-flow path where the water utilized for the water vapor circulates; (f) combustion gas flow path where the combustion gas of combustor after heat-exchanging circulates are formed in the triple pipe structure provided adjacent to coaxial shape; and (g) waste gas cooler by which the CO selective-oxidation device or the methanation device is positioned adjacent to the water-flow path is provided.    An INDEPENDENT CLAIM is a fuel cell system comprising the modification unit; an oxygen-containing gas supply means to supply oxygen-containing gas; fuel cell generated using the oxygen-containing gas supplied by the reformed gas; and oxygen-containing gas supply means which is generated in modification unit.