To provide a new method and a new apparatus for stabilizing incineration fly ash, in each of which inexpensive and highly-stable iron salt is used to prevent the elution of heavy metals.

This method for stabilizing incineration fly ash comprises the steps of: kneading incineration fly ash 5, iron salt or an organic chelate heavy metal fixing agent 12 and water 11 to obtain a kneaded material; and aging 9 the kneaded material in the environment of 40-150°C and ≥40% relative humidity while bringing the kneaded material into contact with the atmosphere. An alkaline component is preferably incorporated in the kneaded material by 0.5-4.0 molar ratio of the alkaline component to the iron salt. The amount of the alkaline component in kneading can be calculated by using the acid consumption (pH8.3) of an eluate from the kneaded material. The iron salt is a mixture of ferrous salt and ferric salt and the molar ratio [Fe2+/(Fe2++Fe3+)]of ferrous salt (Fe2+) to (ferrous salt (Fe2+)+ferric salt (Fe3+)) is preferably 0.2-1.0.

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