To provide a method in which detoxification treatment is stably applied to a harmful heavy metal contained in high alkaline dust collection ashes at a low cost without using an organic or inorganic chemical.

The detoxification treatment method is frequently set up with the aim of effectively collecting hydrogen chloride gas, sulfur dioxide, soot and dust and dioxin or the like which are generated from an incinerator. After slaked lime is blown into the exhaust gas, detoxification treatment is applied to high alkaline dust collection ashes collected in a bag filter. In this case, water is added to dust collection ashes and the mixture is mixed and agitated and thereafter heated at 200-600°C in the atmosphere of air or carbon dioxide. Thereby, detoxification treatment is applied to easily eluted heavy metal, especially lead and zine. The loads of water are ≥25 pts.wt. for 100 pts.wt. of the dust collection ashes. Exhaust gas of the incinerator may be introduced into the above-mentioned atmosphere.