An apparatus for treating an exhaust gas that includes a pre-treatment section that removes a powdery component, a water-soluble component or a hydrolytic component from the exhaust gas. The exhaust gas contains a fluorine compound and CO. A heating oxidative decomposing section performs heating oxidative decomposition of at least one of the fluorine compound and CO to detoxify the exhaust gas. The apparatus also has a post-treatment section for post-treating an acid gas such as HF which has been produced by the heating oxidative decomposition.