To cool exhaust gas while preventing dioxins therein once decomposed at a high temperature from being synthesized again and facilitate the recovery of heat.

In a process of decomposing dioxins for treating wastes containing dioxins at a high temperature, high temperature exhaust gas generated during the high temperature treatment is cooled to below a temperature lower than the resynthesizing temperature of dioxins in a cooling chamber of steam atmosphere or inactive gas atmosphere such as nitrogen. In a process of decomposing dioxins for treating wastes containing dioxins by the melt method, high temperature exhaust gas generated during the high temperature treatment is cooled to below a temperature lower than the resynthesizing temperature of dioxins in a cooling chamber of steam atmosphere or inactive gas atmosphere such as nitrogen. A heat recovering means such as a heat exchanger and the like is used for cooling.