A method of decomposing and removing dioxins from fly ash at a lower temperature in a short time, which comprises contacting the dioxins or the substance containing the dioxins with an amine compound and/or an ammonium compound at a temperature below 300 DEG C. Since the chlorine in the dioxins reacts with the amine compound and/or ammonium compound, the dechlorination of the dioxins proceeds rapidly at a lower temperature. Thus, dioxins can be decomposed and removed in a short time at a low temperature at which dioxins have not been thought to be decomposable.