Carbon monoxide contained in reformate gas is removed by a preferential oxidation reaction in a catalyst, two preferential oxidation reactors (20A, 20B) being disposed in series. Valves (7, 8) supply air containing oxygen as an oxidizing agent individually to these preferential oxidation reactors (20A, 20B). Temperature sensors (9, 10) detect the catalyst temperatures of the preferential oxidation reactors (20A, 20B), and a controller (5), by adjusting the flow rate of the valves (7, 8) based on the detected temperatures, maximizes the carbon monoxide removal performance of the preferential oxidation reactors (20A, 20B), while preventing excessive catalyst temperature rise.