NOVELTY - A carbon monoxide removal catalyst (1) of inducing carbon monoxide shift reaction for allowing water and carbon monoxide to react to produce hydrogen and carbon dioxide, comprises a catalyst carrier having a cerium-zirconium composite oxide and a zirconium oxide; and a predetermined noble metal supported on the catalyst carrier.    USE - The inventive CO removal catalyst is used in a hydrogen purifying device of fuel cell system (claimed) for household or other purposes.    ADVANTAGE - The inventive CO removal catalyst does not undergo great deterioration even when the ambient temperature is as high as 500 degrees C during the starting of the hydrogen purifying device. It has a high heat resistance and does not need to take much time to undergo reduction for inhibiting the heat generation due to reaction heat caused by the reduction reaction as in the copper-based catalyst. Even when air enters the catalyst while the operation of the device is suspended, the catalyst undergoes less deterioration than the copper-based catalyst.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (a) a method of producing a carbon monoxide (CO) removal catalyst, comprising adding zirconia sol to form the cerium-zirconia composite oxide, calcining the cerium-zirconium composite oxide to produce the catalyst carrier, and supporting the predetermined noble metal on the catalyst;    (b) a hydrogen purifying device comprising a reactor (2) filled with a CO removal catalyst of removing CO from a modified gas containing at least CO and water vapor; and    (c) a fuel cell system comprising a hydrogen purifying device, and a fuel cell of generating electricity using a modified gas from which CO has been removed.    DESCRIPTION OF DRAWING(S) - The figure is a schematic longitudinal sectional view of the structure of the hydrogen purifying device of the invention.    Catalyst (1)    Reactor (2)    Modified gas inlet (3)    Modified gas outlet (4)    Diffuser plate (5)    Heat insulating material (6)