In a fuel cell power plant, carbon monoxide contained in reformate gas which is produced by a reformer (3) by means of a catalytic fuel reaction is removed by the respective catalytic reactions of a shift converter (4) and a preferential oxidation reactor (5). During start up of the fuel cell power plant, a burner (6) burns fuel to produce combustion gas and the combustion gas is supplied individually to the reformer (3), shift converter (4) and preferential oxidation reactor (5) through combustion gas supply passages (71-73) thereby simultaneously completing warming up of these catalytic reactors (3-5).