TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Components: The carbon monoxide oxidizer includes an oxidant gas-supply unit having an orifice that reduces the cross-sectional area of a flow of the reformate gas, and a blowout hole spurting out the oxidant gas toward the reformate gas passing through the orifice.The blowout hole is formed at a position offset from a centerline of the orifice to cause the oxidant gas to form a rotating flow in the orifice. A rotating direction of the oxidant gas produced by the blowout holes is set to be inverse to the rotating direction of the mixed gas in the stacked body.The oxidant gas-supply unit comprises blowout holes selectively used based on a flow amount of the reformate gas.The mixing unit includes a porous body carrying carbon monoxide preferential oxidation catalyst through which the mixed gas flows, an oxidant gas-introducing passage formed inside the stacked body for supplying oxidant gas to the rotating passage, a chamber between the oxidant supply unit and stacked body, and a guide arranged inside the chamber to refract a flow of the mixed gas flowing from the orifice toward the stacked body.The porous body is located downstream of the stacked body.The stacked body comprises rotating passages having different cross-sectional areas and designed to rotate the mixed gas in different directions from each other.The mixing unit may comprise stacked bodies arranged in series in a flow direction of the mixed gas. One of the stacked bodies comprises a rotating passage rotating in a direction different from a rotating direction of a rotating passage in an adjacent stacked body.The stacked bodies respectively comprise a rotating passage having different size. Each through-hole is formed so that a size of a through-hole is different from the size of an adjacent through hole and a forming position of a through-hole is different from the forming position of an adjacent through-hole. Each through-hole is formed so that its width expands in a radial direction from a center of each plate. Each plate comprises a passage that circulates a cooling medium.Preferred Dimensions: The size of a through-hole increases as the mixed gas flows downstream. The thickness of plates increases as the mixed gas flows downstream.The chamber has a larger cross-sectional area than the cross-sectional area of the orifice.