NOVELTY - A carbon monoxide oxidizer comprises a catalyst for promoting oxidizing of carbon monoxide; a main passage that allows flow of reformate gas from a reformer to an agitator; an inlet upstream of the catalyst that supplies oxidizing agent to the reformate gas; and agitator for agitating the reformate from the main passage.    USE - For a fuel cell system.    ADVANTAGE - The inventive carbon monoxide oxidizer has an agitator that allows uniform agitation in a compact structure and that avoids large pressure loss.    DETAILED DESCRIPTION - A carbon monoxide oxidizer (1) is connected to a reformer (2) performing reformate reactions to produce a reformate gas containing hydrogen and carbon monoxide. It has a catalyst (6) for promoting oxidizing of carbon monoxide to reduce a concentration of carbon monoxide in the reformate gas produced by the reformer; a main passage that allows flow of reformate gas supplied from the reformer to an agitator (4); an inlet (5) disposed upstream of the catalyst that supplies oxidizing agent to the reformate gas; and agitator for agitating the reformate from the main passage. The agitator comprises at least 3 members that impede a main stream of reformate gas flowing through the main passage. Adjacent members of the at least 3 members are disposed so that their cross sections, projected in the direction of the main stream, partially overlap so that their cross-sections projected in the direction perpendicular to the main stream flowing in the main passage partially overlap. The curved passages have at least 2 curves formed downstream on the inlet and between the at least 3 members. The agitator divides the main stream flowing through the main passage into sub-streams flowing through the curved passage.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram showing the structure of a carbon monoxide oxidizer in a fuel cell system.    Carbon monoxide oxidizer (1)    Reformer (2)    Agitator (4)    Inlet (5)    Catalyst (6)