To make it possible to simply and effectively adsorb and remove a harmful substance such as dioxins from an exhaust gas by preparing a treatment agent for treating the exhaust gas containing a harmful organic matter such as dioxins, of a carbon black alone or the carbon black as a main constituent.

An exhaust gas treatment agent is prepared using a carbon black alone or the carbon black as a main constituent and the carbon black to be used is preferably of a powder form whose average particle diameter is 1 μm or less. The exhaust gas treatment agent is blown into a flue of an incineration installation in an exhaust gas treatment step and is recovered by using an electric dust collector, a bag filter or the like. Thus, a harmful organic matter such as dioxins is removed from the exhaust gas. The position into which the exhaust gas treatment agent is blown is preferably a part where a gas temperature is about 300°C or less, that is, across the stretch from a stop immediately after a heat exchanger to the front stage of the electric dust collector, especially right after the outlet of the heat exchanger, as the ignition point of the carbon black is about 350°C.