PURPOSE: To confine heavy metals in a stable state into solidified ash and to prevent the elution of the heavy metals from the solidified ash by removing water-soluble salts which is a destruction factor of the solidified ash, insolubilizing the heavy metals, and forming strong solidified ash by a hydrothermal reaction.

CONSTITUTION: In the treatment of incinerated fly ash 1, the ash 1 is introduced to a fly ash washer 2 to be washed with washing water 3, water soluble salts contained in the ash 1 are dissolved in the washing water 3, and the salt water 4 in which the water soluble salts are dissolved is discharged outside a system. In this way, the obtained desalted ash 5 is incorporated with an appropriate amount of water 6 and a heavy metal insolbilizing agent 7 such as NaHS, and the heavy metals contained in the ash 5 are converted into a sparingly water-soluble compounds such as sulfide. The ash 5 containing the sparingly water-soluble heavy metal compounds is introduced to a heating compressor 8 to be heated and compressed, and a hydrothermal reaction is generated to produce strong solidified ash. In this way, the heavy metals in a stable state are confined into the strong solidified ash 9, hindering the elution of the heavy metals from the solidified ash 9.