To provide a treatment method and a device of combustion exhaust gas which reduce a percentage content of dioxins in a collected fly ash, and surely decompose dioxins in exhaust gas by a subsequent dioxin decomposition device.SOLUTION: A treatment method of combustion exhaust gas includes a collecting step of collecting mercury in exhaust gas and dust using an adsorbent that hardly adsorbs dioxins, and a decomposition step of decomposing the dioxins in the exhaust gas having passed through the collecting step, in which in the collection step, activated carbon having a pore of 2 nm or smaller is used.SELECTED DRAWING: Figure 1