NOVELTY - A hydrogen manufacturing system has a modifier (2), carbon monoxide (CO) transformers (3,5), a CO selective-oxidation device (6) and adsorbers (4a,4b). The modifier obtains reformed gas by modifying hydrocarbon fuel. The transformers (3,5) and the device (6) decrease CO concentration in reformed gas. The adsorbers equipped with adsorption agent, adsorbs a decomposed residual component to downstream of modifier.    USE - For fuel cell system (claimed) used for electric power generation.    ADVANTAGE - The hydrogen manufacturing system is stable and efficient, even when a decomposed residual component exists in the reformed gas. The fuel cell system is also efficient.    DETAILED DESCRIPTION - A hydrogen manufacturing system has a modifier (2), carbon monoxide (CO) transformers (3,5), a CO selective-oxidation device (6) and adsorbers (4a,4b). The modifier obtains reformed gas containing hydrogen, by modifying a hydrocarbon fuel. The transformers (3,5) decrease CO concentration in the reformed gas by a shift reaction. The device (6) further decreases CO concentration in the gas in which carbon monoxide concentration is decreased by the CO transformer, by selective oxidation. The adsorbers are equipped with adsorption agent for adsorbing a decomposed residual component to the downstream of modifier. An INDEPENDENT CLAIM is included for fuel cell system, which has the hydrogen manufacturing system and a fuel cell (7). The fuel cell utilizes the reformed gas in which CO concentration is decreased by the selective-oxidation device, as a fuel. The adsorber equipped with adsorption agent adsorbs the decomposed residual component to the downstream of modifier and upstream of fuel electrode of fuel cell.    DESCRIPTION OF DRAWING(S) - The figure shows a flowchart of fuel cell system. (Drawing includes non-English language text).    modifier (2)    carbon monoxide transformers (3,5)    adsorbers (4a,4b)    carbon monoxide selective oxidation device (6)    fuel cell (7)    heat exchanger (8)