NOVELTY - The preferential oxidation reactor (50) comprises a first reaction unit (10) including a first internal space that is to be supplied with mixed hydrogen gas and connected with a first opening portion (12) and a first catalyst capable of spontaneously igniting hydrogen mixed gas at a predetermined temperature of the first internal space, a second reaction unit (30) including a second internal space connected to a second opening portion and a second catalyst contained in the second internal space, and a separation layer positioned between the first and second catalysts.    USE - The preferential oxidation reactor is useful in a fuel cell system.    ADVANTAGE - The preferential oxidation reactor has excellent carbon monoxide reducing performance while providing for self-ignition of mixed hydrogen gas, and can be economically manufactured with reduced size and improved efficiency.    DETAILED DESCRIPTION - The preferential oxidation reactor (50) comprises a first reaction unit (10) including a first internal space that is to be supplied with mixed hydrogen gas and connected with a first opening portion (12) and a first catalyst capable of spontaneously igniting hydrogen mixed gas at a predetermined temperature of the first internal space, a second reaction unit (30) including a second internal space connected to a second opening portion and a second catalyst contained in the second internal space, a separation layer positioned between the first and second catalysts to separate the first and second catalyst, a water receiving unit provided lower than the first reaction unit in a direction of gravitational force field lines, and a discharge unit that connects the water receiving unit and the second opening portion to each other to be in fluid communication. The separation layer connects the first and second internal spaces to be in fluid communication, and includes a porous plate member. An entire volume of the first catalyst is greater than or equal to 0.004 times the flow rate per minute of hydrogen mixed gas supplied to the first internal space. The first and second catalysts contain a ceramic carrier, a metallic carrier and/or a carbon carrier having a granule shape. The second catalyst includes a granule-type carrier larger than the carrier of the first catalyst, and a non-metal. The second reaction unit is positioned lower than the first reaction unit in the direction of gravitational force field lines. The second opening portion is positioned in an upper part of the water receiving unit in the direction of gravitational force field lines. The first reaction unit, the second reaction unit and the discharge unit have a dual-pipe structure.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a preferential oxidation reactor.    First reaction unit (10)    Housing (11)    First opening portion (12)    Second reaction unit (30)    Preferential oxidation reactor. (50)