NOVELTY - A fuel cell reformer (100) has modification unit (120), carbon monoxide removal unit (140), oxygen-containing gas supply section, catalyst layer temperature detection unit (142) and a control apparatus (180). The modification unit generates reformed gas which reforms raw fuel and has hydrogen gas as main component. The carbon monoxide removal unit has a catalyst layer containing selective oxidation catalyst, and makes reformed gas and oxygen-containing gas contact in presence of selective-oxidation catalyst and selectively oxidizes carbon monoxide in reformed gas.    USE - Reformer is used for fuel cell.    ADVANTAGE - The selective-oxidation catalyst layer of carbon monoxide removal unit in fuel cell reformer is maintained at favorable temperature. The fuel cell system comprising the reformer has improved efficiency.    DETAILED DESCRIPTION - A fuel cell reformer has modification unit, carbon monoxide removal unit, oxygen-containing gas supply section, catalyst layer temperature detection unit and a control apparatus. The modification unit generates reformed gas which reforms raw fuel and has hydrogen gas as main component. The carbon monoxide removal unit has a catalyst layer containing selective oxidation catalyst, and makes reformed gas and oxygen-containing gas contact in presence of selective-oxidation catalyst and selectively oxidizes carbon monoxide in reformed gas. The oxygen-containing gas supply section supplies oxygen-containing gas to carbon monoxide removal unit. The catalyst layer temperature detection unit detects temperature of catalyst layer. The control apparatus is controlled to approach the temperature of catalyst layer during selective oxidation reaction close to target temperature appropriate to selective-oxidation reaction, by controlling the oxygen-containing gas supply section according to detection result of catalyst layer temperature detection unit, and adjusting feed rate of oxygen-containing gas per unit time.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of fuel cell system. (Drawing includes non-English language text)    Fuel cell system (10)    Fuel cell reformer (100)    Desulfurization unit (110)    Modification unit (120)    Carbon monoxide conversion unit (130)    Carbon monoxide removal unit (140)    Catalyst layer temperature detection unit (142)    Control apparatus (180)