The invention discloses a CO shift catalyst, a CO shift reaction apparatus and a method for purifying gasified gas. The CO shift catalyst is a CO shift catalyst for modifying carbon monoxide (CO) in a gas, which comprises: an active component that contains either molybdenum (Mo) or iron (Fe) as a main component, while containing either nickel (Ni) or ruthenium (Ru) as a sub-component; and a carrier that supports the active component and is formed of an oxide of one or more elements selected from among titanium (Ti), zirconium (Zr) and cerium (Ce). Catalyst production firing for this CO shift catalyst is carried out at 550 DEG C or more.