A reformer (8) reforms hydrocarbon fuel and generates reformate gas. A first carbon monoxide oxidizer (1) and second carbon monoxide oxidizer (2) disposed in series decrease the carbon monoxide concentration of the reformate gas by a prefential oxidation. The air for the preferential oxidation is supplied to the carbon monoxide oxidizer (1, 2) from a compressor (9). The reformer (8) consumes the water heated by the reaction heat of the first carbon monoxide oxidizer (1). The water amount supplied for the reformer (8) increases as a fuel reforming requirement of the reformer (8) increases. When the fuel reforming requirement increases, the water heating capability of the first carbon monoxide oxidizer (1) is enhanced with a sufficient response by increasing the proportion of air supplied to the first carbon monoxide oxidizer (1).