To detoxify dioxins in waste water efficiently in large quantities although a small-scale apparatus is used.

A method for detoxifying dioxins comprises: a step S1 of dissolving dioxin-containing soot and dust in the generated gas, chlorine and oxidized sulfur in a magnesium hydroxide solution (circulating water) circulating in a desulfurization column and recovering the resulting circulation water; a step S2 of bringing the recovered circulation water into contact with a nonpolar solvent such as hexane to extract dioxins in the nonpolar solvent; a step S3 of concentrating the extracted dioxin-containing nonpolar solvent in a vacuum distillation apparatus; a step S4 of charging the dioxin-concentrated nonpolar solvent into alcohol such as ethanol; a step S5 of adding a proper quantity of metal calcium or metal magnesium to the resulting alcohol; and a step S6 of irradiating the metal calcium or metal magnesium-added alcohol with ultraviolet rays from a low-pressure mercury lamp while agitating the metal calcium or metal magnesium-added alcohol to decompose the dioxins in the metal calcium or metal magnesium-added alcohol.