The present invention provides a catalyst precursor substance containing copper, zinc, and aluminum and exhibiting an X-ray diffraction pattern having a broad peak at a specific interplanar spacing d (Å). The present invention also provides a method for producing the catalyst precursor substance by mixing a solution containing a copper salt, a zinc salt, and an aluminum salt with a solution containing an alkali metal hydroxide or an alkaline earth metal hydroxide, thereby forming a precipitate. In the present invention, a catalyst is prepared through calcining of the catalyst precursor; the catalyst is employed for water gas shift reaction; and carbon monoxide conversion is carried out by use of the catalyst. Thus, the present invention also provides a catalyst useful for water gas shift reaction, which exhibits high activity and durability and which, even when applied to a fuel cell, can be used for a long period of time with reduction in activity being suppressed; a carbon monoxide conversion method employing the catalyst; and a fuel cell system employing hydrogen produced through the carbon monoxide conversion method.