To provide a method for making exhaust gas from an incinerator harmless, capable of advantageously being applied to a small-sized incinerator.

The treatment method for making the exhaust gas from an incinerator hamless includes a cooling process for cooling a high temperature exhaust gas containing fly ash discharged from the incinerator by brining the gas into contact with a cooling liquid, a solid-gas separation process for introducing the cooled exhaust gas into the solid-gas separation device and separating the fly ash contained in the exhaust gas, and a gas-liquid contact process for introducing the exhaust gas from which the fly ash obtained by the solid-gas separation process is removed into the gas-liquid separation device to be brought into contact with a treating liquid, and for collecting the fly ash remaining in the exhaust gas in the treating liquid to be separated. The treating liquid containing the fly ash obtained by the gas-liquid contact process, is used as at least one portion of the cooling liquid.