We succeeded in detoxification of hexachloro-benzene adsorbed on artificially produced fly ash in air by irradiating microwave (2.45 GHz) in the presence of activated carbon powder. Hexachlorobenzene was decomposed by 50-90% at 200-300 by MW irradiation of 1-1.5 min when the ash contained activated carbon by 12 wt% and water by 10 wt%. Chlorinated benzene derivatives are dechlorinated through substitution of chloride anion with hydroxylation produced by basic CaO in the co-presence of activated carbon effectively heated by MW. This method using microwave irradiation enables us to treat the contaminated fly ash in a shorter time and decompose hexachlorobenzene more efficiently than the conventional heating.