To provide a combustion gas treatment method for efficiently and economically removing heavy metals such as mercury from an exhaust combustion gas containing heavy metals.

In a treatment method for an exhaust combustion gas containing heavy metals and discharged out an incinerator for wastes, before dust collection treatment is carried out by a highly efficient dust collecting apparatus, an incinerator ash slurry produced from a heavy metal stabilization agent, incinerator ashes and water is sprayed to the exhaust combustion gas to adsorb the heavy metals of the exhaust combustion gas in the slurry. Alternatively, prior to the foregoing treatment, the exhaust combustion gas containing heavy metals and discharged out an incinerator is subjected to dust collection treatment by a hot cyclone or a ceramic filter at a temperature not lower than the boiling point of the heavy metals as to take out incinerator ashes having lower harmfulness without transferring heavy metals with low boiling points to the incinerator ashes.