To produce a catalyst for selectively oxidizing carbon monoxide from a raw material gas containing carbon monoxide and hydrogen, to provide an apparatus for decreasing the carbon monoxide concentration using the catalyst, and a fuel cell system using the apparatus.

Carbon monoxide in the raw material gas containing carbon monoxide and hydrogen is selectively oxidized and the carbon monoxide concentration is decreased by using the catalyst carrying ruthenium, comprising a support of an inorganic oxide containing at least one of alumina, silica, zirconia, and titania and ruthenium on the support and obtained by bringing the support and a solution containing sodium hydroxide and/or potassium hydroxide together with sodium ruthenate and/or potassium ruthenate into contact with each other.