To provide a treating agent which can reduce the amount of dioxins generated in collected dust and ash without decreasing the capability for adsorbing dioxins in an exhaust gas by combining an active carbon powder excellent in capability for adsorbing and removing dioxins with a carbonaceous powder having no unnecessarily fine micropore.

A carbon powder prepared by mixing an active carbon powder having a specific surface area of 500 m2/g or higher with a carbonaceous powder having a specific surface area of 100 m2/g or lower in a wt, ratio of (8/2)-(2/8) or by mixing an active carbon powder of which the volume of micropores having diameters of 20 or lower is 0.2 ml/g or higher with a carbonaceous powder of which the volume of micropores having diameters of 20 or lower is 0.09 ml/g or lower in a wt ratio of (8/2)-(2/8) is used. Thus, gaseous dioxins in an exhaust gas can be efficiently adsorbed and removed and, since precursors of dioxins are scarcely adsorbed, the generation of dioxins in a dust collector can be inhibited.