Conversions of acetaldehyde and trimethylamine as model unpleasant odor gases were measured over several catalysts. The activities of Ag and Mn2O3 were the highest of the catalysts that consisted of a single component. The addition of Ag to manganese oxide enhanced the activity and the durability of the catalysts. The activity of Mn catalyst was improved by the addition of Ag UP to 40 atom-%, and the maximum activity was observed at 10 atom-%. Acetaldehyde was considered to be decomposed over the Ag-Mn catalyst by oxidation according to the measurement of reaction products. The oxidation state of silver was maintained on the surface of the AS-Mn catalyst calcined at 773 K. The amount of oxygen adsorbed on the surface of the AS-Mn catalyst was about 2.9 times as much as that on Mn2O3. These experimental data suggested that manganese oxide supplied oxygen to silver and that the oxidation state of silver was maintained on the surface of the Ag-Mn catalyst.