To provide a method for treating an incineration ash by which the removing treatment can be carried out at a normal temperature, in a process for removing a hard-to-decompose substance contained in the incineration ash.

In the method for treating the incineration ash by which the hard-to-decompose substance such as dioxins, endocrine disrupters including PCB, which are contained in the incineration ash discharged from a boiler or an incinerator, is removed by oxidative decomposition, the incineration ash is mixed with water into a slurry form. After this slurry formation, the structure of the ash is broken and the hard-to-decompose substance drawn into the interior of the ash is pretreated so that the substance deposits on the surface of the ash. Then the substance is decomposed by oxidation in the presence of a hydroxy radical and a heavy metal removing process is performed.