To provide a processing method of combustion ash which reduces the amount of elution of fluorine or boron included in the combustion ash to the environmental standard value or less.

The processing method of combustion ash comprises the steps of mixing the combustion ash under the presence of calcium oxide and/or calcium hydroxide, alumina cement and an aqueous solution of sulfuric acid to prepare combustion ash having an elution amount of fluorine of 0.8 mg/L or less and an elution amount of boron of 1.0 mg/L or less in accordance with the method of the elution test based on Ministry of Environment announcement 2003, No. 18 when fluorine and/or boron including the combustion ash is eluted.