carbon monoxide contained in reformate gas is removed by a preferential oxidation reaction in a catalyst , two preferential oxidation reactors being disposed in series . valves supply air containing oxygen as an oxidizing agent individually to these preferential oxidation reactors . temperature sensors detect the catalyst temperatures of the preferential oxidation reactors , and a controller , by adjusting the flow rate of the valves based on the detected temperatures , maximizes the carbon monoxide removal performance of the preferential oxidation reactors , while preventing excessive catalyst temperature rise .