To provide a method capable of simultaneously performing detoxification treatment by adding a chemical to a harmful substance such as a harmful heavy metal and an organic chlorine-based compound contained in dust collection ashes discharged from an incinerator and in residual ashes of the incinerator and heating the mixture at low temperature.

In the detoxification treatment method, water and an inorganic acid hydrogen salt compound are added to dust collection ashes and incineration residual ashes and the mixture is agitated and thereafter heated in the temperature range of 200-600°C in the oxidative atmosphere and thereafter rapidly cooled at ≤100°C. The inorganic acid hydrogen salt compound may be added in a powdery state or a liquid state. At least one kind selected from among a phosphoric acid hydrogen salt compound, a carbonic acid hydrogen salt compound and a sulfuric acid hydrogen salt compound is desirably used. Further, the atmosphere at time of heat treatment favorably is at least one kind selected from air, carbonic acid gas and gas obtained after dehydrochlorination of exhaust gas of an incinerator. The cost of equipment and the installation site of a facility are made smaller by this treating method and treatment cost is made very inexpensive.