There is disclosed a COS treatment apparatus for a gasified gas, in which 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 are provided. Also, there is disclosed a COS treatment apparatus containing a TiO.sub.2 catalyst that carries Cr.sub.2O.sub.3 or NiO. Further, there is disclosed a COS treatment method including 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.