An oxidation catalyst for purifying an exhaust gas, which can provide an excellent catalyst activity at lower temperatures for particulates and high boiling point hydrocarbons in an exhaust gas of an internal combustion engine, is provided. The oxidation catalyst for purifying an exhaust gas is composed of a composite metal oxide represented by the general formula: Y.sub.1-xAg.sub.xMnO.sub.3, wherein 0.01.ltoreq.x.ltoreq.0.15. The composite metal oxide is represented by the general formula: Y.sub.1-xAg.sub.xMn.sub.1-yA.sub.yO.sub.3, wherein A is one metal selected from the group consisting of Ti, Nb, Ta and Ru, and 0.005.ltoreq.y.ltoreq.0.2.