NOVELTY - A carbon monoxide selective oxidizing catalyst comprises a carrier consisting of ferrierite particles and/or ZSM-5 particles having a largest pore having a maximum pore diameter of 0.55-0.65 nm; and metal component that is supported on the carrier. When the catalyst receives a supply of a hydrogen-rich gas containing carbon monoxide, the catalyst promotes a carbon monoxide selective oxidizing reaction.    USE - For carbon monoxide concentration reduction apparatus for fuel cell system (claimed).    ADVANTAGE - The carbon monoxide selective oxidizing catalyst achieves a higher carbon monoxide concentration reduction rate, provides higher selectivity in oxidizing carbon monoxide, and exhibits sufficient activity at lower temperatures to improve performance of the entire fuel cell system. It reduces the concentration of carbon monoxide contained in a hydrogen-rich gas that contains carbon monoxide, like a reformed gas.    DETAILED DESCRIPTION - A carbon monoxide selective oxidizing catalyst comprises a carrier consisting of ferrierite particles and/or ZSM-5 particles, the ferrierite and ZSM-5 particles having a largest pore having a maximum pore diameter of 0.55-0.65 nm; and metal component that is supported on the carrier and that includes a component consisting of platinum alone and platinum and type(s) of transition metal. When the carbon monoxide selective oxidizing catalyst receives a supply of a hydrogen-rich gas containing carbon monoxide, the carbon monoxide selective oxidizing catalyst promotes a carbon monoxide selective oxidizing reaction that oxidizes carbon monoxide by giving carbon monoxide priority over hydrogen. An INDEPENDENT CLAIM is also included for a method of manufacturing a carbon monoxide selective oxidizing catalyst that receives a supply of a hydrogen-rich gas containing carbon monoxide and promotes a carbon monoxide selective oxidizing reaction that oxidizes carbon monoxide by giving carbon monoxide priority over hydrogen, comprising selecting particles of ferrierite particles and/or ZSM-5 particles; and preparing a carrier consisting of the selected particles; and supporting on the prepared carrier a metal component that is supported on the carrier.