To inexpensively treat coal ash discharged from a coal firing boiler into an easily treatable form suitable for reclamation or the like.

The calcium oxide-coating coal ash discharged from the coal firing boiler such as a pressure fluidized bed type boiler is a material suitable as the object for consolidation by carbonation treatment. Thus, moisture is incorporated into a granular raw material including the calcium oxide-containing coal ash as a part of the raw material at least, then the moisture-containing granular raw material is brought into a carbonation reaction in the presence of carbon dioxide, thereby consolidating the raw material particles of the calcium oxide-containing coal ash properly with each other.