A carbon monoxide oxidizer comprises an oxidant supply unit (4) that supplies an oxidant gas to a reformate gas, a mixing unit (5) that mixes the reformate gas with the oxidant gas, and a preferential oxidation catalyst unit (6) that removes carbon monoxide from a mixed gas using catalytic action. The mixing unit (5) comprises a stacked body (9) of plates (100A-100F). A rotating passage (101) that is formed in the stacked body (9) to rotate the flow of the mixed gas, thereby promoting mixing of the reformate gas with the oxidant gas by a rotating flow formed by the rotating passage (101).