The present invention relates to a process for treating a dioxin-containing exhaust gas which can inhibit the generation of dioxin occurring either in the course of cooling of the exhaust gas between a downstream portion subsequent to a combustion chamber and a dust collector or due to a memory effect upon low-temperature combustion in flues or dust collector at the start-up or shut-down of intermittent operation-type solid waste incinerators, without large-scale incineration facilities or plant renovation and equipment investment, and a composite catalyst composition for inhibiting the generation of dioxin, comprising an iron compound, an acid gas neutralizing agent and optionally an activated carbon.