A CO shift catalyst according to the present invention reforms carbon monoxide (CO) in gas. The CO shift catalyst has one of molybdenum (Mo) or iron (Fe) as a main component and has an active ingredient having one of nickel (Ni) or ruthenium (Ru) as an accessory component and one or two or more kinds of oxides from among titanium (Ti), zirconium (Zr), and cerium (Ce) for supporting the active ingredient as a support. The temperature at the time of manufacturing and firing the catalyst is equal to or higher than 550° C.