NOVELTY - The preferential oxidation reactor comprises a housing (2) having a catalytic layer (3), a mixture gas supply portion that is inserted into an interior of the housing and penetrates the catalytic layer, a condensate receiving portion (4) that is contained within the housing and is present in fluid communication with a gas outlet port of the mixture gas supply portion, condensate control tubes that are arranged in the catalytic layer of the housing and are present in fluid communication with the condensate receiving portion, and a partition plate.    USE - The preferential oxidation reactor is useful in a fuel cell system (claimed), and for removing carbon monoxide in a hydrogen mixture.    ADVANTAGE - The preferential oxidation reactor prevents the delay in the catalytic reaction due to the overflow of condensate water, and improves the cooling efficiency of the catalysts.    DETAILED DESCRIPTION - The preferential oxidation reactor comprises a housing (2) having a catalytic layer (3), a mixture gas supply portion that is inserted into an interior of the housing and penetrates the catalytic layer, a condensate receiving portion (4) that is contained within the housing and is present in fluid communication with a gas outlet port of the mixture gas supply portion, condensate control tubes that are arranged in the catalytic layer of the housing and are present in fluid communication with the condensate receiving portion, and a partition plate between the catalytic layer and the condensate receiving portion. The condensate control tubes: have a capillary structure; are positioned upright from the condensate receiving portion such that lower portions of the condensate control tubes are placed within the condensate receiving portion; are arranged along an inner circumferential surface of an inner wall of the housing; and are adhered to the inner wall of the housing. Upper portions of the condensate control tubes penetrate a top of the catalytic layer. The partition plate comprises a porous structure, and is positioned such that the mixture gas supply portion and the condensate control tubes penetrate the partition plate. An INDEPENDENT CLAIM is included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an internal structure of a preferential oxidation reactor.    Supply tube (1)    Housing (2)    Catalytic layer (3)    Condensate receiving portion (4)    Discharge port. (5)