A COS treatment apparatus for a gasified gas includes an O.sub.2 removal catalyst and a COS conversion catalyst located on the downstream side of a gasified gas flow with respect to the O.sub.2 removal catalyst. Also, a COS treatment apparatus includes a TiO.sub.2 catalyst that carries Cr.sub.2O.sub.3 or NiO. Further, a COS treatment method includes a first step in which O.sub.2 is removed by the reaction with H.sub.2S and CO, and a second step in which COS is converted to H.sub.2S.