In MSWI, various treatment processes are performed for decreasing fly ash, acid gas, heavy metals and dioxins. Nowadays, the review of each processes and the development of new technologies are done because it is needed to restraint emission of environmental pollutant substances and optimization of running cost and energy in the flue gas treatment. As one of them, the high temperature dust collection using the ceramic filter is expected to decrease running cost and energy in the plant, owing to omit the process of gas cooling and reheating. In this study, it was investigated that the decomposition availability of dioxins by dechlorination using activated clays as inorganic matters substituted for activated carbon at 300^oC, the dust collecting operation temperature. As the basic study of it, using activated clays made in Japan, it was confirmed the removal rate of mono-chlorobenzene, the removal special characteristic and the effect of dechlorination, depending on temperature conditions.As a result, activated clays are divided according to their ability of dechlorination.