NOVELTY - A carbon monoxide oxidizing catalyst (C1) for a reformer of a fuel cell system comprises: a carrier selected from alumina and/or cordierite; and an active material including cerium oxide, a transition element (M) oxide, and copper oxide. The transition element (M) is supported on the carrier.    USE - As a carbon monoxide oxidizing catalyst for a reformer of a fuel cell system (claimed), useful for producing electrical energy.    ADVANTAGE - The catalyst exhibits excellent carbon monoxide oxidation efficiency for a reformer of a fuel cell system.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) preparing a carbon monoxide (CO) oxidizing catalyst (C1) involving: mixing alumina and/or cordierite with a cerium compound solution; adding the mixture to a transition element compound solution; subjecting the mixture to a first calcination to form a calcinated product; adding a copper compound solution to the calcinated product to form a mixture; and subjecting the mixture to a second calcination; and    (2) a fuel cell system (100) comprising: a reformer (30), which includes a reforming reaction part (32) that generates hydrogen gas from a fuel by a catalyst reforming reaction requiring heat energy, and a carbon monoxide reducing part (33), which reduces the (CO) concentration in the hydrogen gas through oxidation reaction of hydrogen with oxygen; at least one electricity generation element (11) for generating electrical energy by electrochemical reactions of hydrogen gas with oxygen; a fuel supplier (50) for supplying the fuel to the reforming reaction part; an oxidant supplier (70) for supplying oxidant to the CO reducing part and at least one electricity generating element; and a cooler for cooling heat generated by the CO reducing part by circulating the fuel supplied to the reforming reaction part to the CO reducing part; where the reformer further comprises a carrier selected from alumina and/or cordierite; and an active material including cerium oxide, a transition element oxide (MO), (where the transition element (M) is supported on the carrier), and copper oxide (CuO).    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of a fuel cell system.    electricity generation element (11)    reformer (30)    reforming reaction part (32)    carbon monoxide reducing part (33)    fuel supplier (50)    oxidant supplier. (70)