NOVELTY - An operation-control method of a reformed-gas manufacturing apparatus, where the apparatus comprises: a modifier (620) which generates the reformed gas which has hydrogen gas as a main component; a carbon monoxide (CO)-modifying device (810) to which the reformed gas generated with the modifier is supplied; a CO selective-oxidation device (830); a methanation device; and an inflow chamber (832) by which the reformed gas flows in.    USE - The method is useful for operation-control of a reformed-gas manufacturing apparatus (claimed).    ADVANTAGE - The method is simple and stable, and efficiently suppresses catalyst deterioration.    DETAILED DESCRIPTION - An operation-control method of the reformed-gas manufacturing apparatus, where the apparatus comprises: a modifier which generates the reformed gas which has hydrogen gas as a main component, where steam reforming process of the source gas containing hydrocarbon fuel and water vapor is carried out by contacting with a modification catalyst in heating atmosphere; a carbon monoxide (CO)-modifying device to which the reformed gas generated with the modifier is supplied, and which transforms CO in the reformed gas to a carbon dioxide (CO2) with a CO-modifying catalyst; a CO selective-oxidation device which oxidizes CO2 to CO to which the reformed gas processed by this CO-modifying device is supplied, and remains in the reformed gas according to CO selective-oxidation catalyst; a methanation device to which the methanation of the CO is carried out; also, an inflow chamber by which the reformed gas flows in is provided, and a catalyst filling chamber where it connects to the inflow chamber, and fills with a catalyst, is used as the CO-modifying device; when the modification catalyst reaches predetermined temperature at the time of temperature rising of the modifier, the water made to generate the water vapor that can purge the hydrocarbon fuel of the gaseous state with which it fills in the modifier will be supplied, before the water vapor that flows in the inflow chamber condensing and overflowing and flowing in into the catalyst filling chamber interior, supply of the water is stopped. INDEPENDENT CLAIMS are:    (1) a reformed-gas manufacturing apparatus comprising: a modifier which generates the reformed gas which has hydrogen gas as a main component, where steam reforming process of the source gas containing hydrocarbon fuel and water vapor is carried out by contacting with a modification catalyst in heating atmosphere; a CO-modifying device which the reformed gas generated with this modifier is supplied, and transforms CO in the reformed gas to a CO2 according to a CO-modifying catalyst; a CO selective-oxidation device to which the reformed gas processed by this CO-modifying device is supplied, and which oxidizes CO which remains in the reformed gas to CO2 according to CO selective-oxidation catalyst or the methanation device to which the methanation of the CO is carried out; a control apparatus, where the CO-modifying device is equipped with the inflow chamber by which the reformed gas flows in into an inside, and the catalyst filling chamber where it connects to this inflow chamber, and fills with a catalyst; the control apparatus will supply the water made to produce the water vapor that can purge the hydrocarbon fuel of the gaseous state with which it fills in the modifier, when the modification catalyst reaches predetermined temperature at the time of temperature rising of the modifier, and before the water vapor that flows in, in the inflow chamber condensing and overflowing and flowing in into the catalyst filling chamber interior, control which stops supply of the water is carried out; and    (2) a fuel cell system, comprising the reformed-gas manufacturing apparatus, an oxygen-containing gas supply means to supply oxygen-containing gas, a fuel cell generated using the apparatus, an oxygen-containing gas supply means which were generated by the reformed-gas manufacturing apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a side sectional schematic view of a modification unit in a fuel cell system.    Fuel cell system (100)    Burner unit as a combustor (151)    Heat exchanger apparatus which is a heat exchanger (160)    Fuel cell stack (200)    Modification unit as a reformed-gas manufacturing apparatus (400)    Modification part as a modifier (600)    Modifier (620)    CO-modifying device (810)    CO-modifying division board (811)    Gas diffusion region as an inflow chamber (812)    CO-modifying reaction region as a catalyst filling chamber (813)    Heat processing means (820)    CO selective-oxidation device (830)    CO selective-oxidation division board (831)    Diffusion region as an inflow chamber (832)    CO selective-oxidation reaction region as a catalyst filling chamber (833)    Waste gas cooler as a heat exchange part (840)