NOVELTY - A carbon monoxide selective oxidizing catalyst comprises a carrier of ferrierite or ZSM-5 supporting a metal component including platinum (Pt) optionally with transition metal(s). The catalyst receives a supply of a hydrogen-rich gas containing carbon monoxide, and promotes selective oxidation of carbon monoxide by giving the carbon monoxide priority over hydrogen.    USE - For use in fuel cell system (claimed), for reducing the concentration of carbon monoxide in the fuel gas supplied to the fuel cell.    ADVANTAGE - The carbon monoxide selective oxidation catalyst achieves higher carbon monoxide concentration reduction rate, provides higher selectivity in oxidizing carbon monoxide and exhibits sufficient activity at lower temperatures. Since the carbon monoxide reduction is performed at lower temperature, undesirable side reactions such as reverse shift reactions and methanation are suppressed, thereby improving the efficiency of reducing carbon monoxide concentration in the hydrogen-rich gas. Since the efficiency is improved, a more compact apparatus with the catalyst is enabled. The fuel cell system is prevented from being poisoned by carbon monoxide, as carbon monoxide concentration in the hydrogen rich gas is reduced by the catalyst. Therefore, cell performance is maintained at a high level. By setting the maximum pore diameter of the carrier to 0.55-0.65 nm, the catalyst performance is raised. When the carrier is a solid acid, the bond strength between carbon monoxide and platinum is weakened and the catalyst is prevented from being poisoned by carbon monoxide. As platinum is bound with oxygen more easily, the carbon monoxide selective oxidation reaction is further promoted. By setting the molar ratio between platinum and iron to 1.5-7.5, it is possible to perform carbon monoxide selective oxidation at low temperature. Hydrogen-rich gas of high purity is obtained.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) Carbon monoxide concentration reduction apparatus comprising hydrogen-rich gas supply that supplies hydrogen-rich gas, an oxygen supply that supplies oxygen used for oxidizing carbon monoxide, and a carbon monoxide selective oxidizing reactor including the carbon monoxide selective oxidizing catalyst. The apparatus oxidizes CO contained in hydrogen rich gas, thereby reducing CO concentration in hydrogen-rich gas;    (2) A fuel cell system with a fuel cell (18) that receives a supply of fuel gas containing hydrogen and an oxidizing gas containing oxygen, and that obtains electromotive force through an electrochemical reaction. The system comprises a fuel gas supply with the carbon monoxide concentration reduction apparatus, which reduces CO concentration in the hydrogen-rich gas and supplies fuel gas to the fuel cell; and    (3) Method of manufacturing carbon monoxide selective oxidizing catalyst, which involves preparing a carrier comprising ferrierite or ZSM-5 and supporting the metal component on the carrier.    DESCRIPTION OF DRAWING(S) - The figure shows the explanatory drawing of the configuration of fuel cell system.    Fuel cell (18)