To provide a method and apparatus for treating toxic substances in exhaust gas which can treat harmful substances, such as an acidic gas and hardly decomposable substances, with a high removal rate and can realize low-cost treatment through a reduction in the amount of a chemical supplied.

This apparatus for removing toxic substances, such as hardly decomposable substances contained in exhaust gas 20 discharged from an incinerator or the like, includes a means 10 for separating fly ash 22 from the waste gas 20, a mixing means 12, in which an acid and water are added to the fly ash 22 to slurry the fly ash 22, and a means 14 for oxidatively decomposing the slurry in the presence of hydroxy radicals and further includes a first scrubber 11a for spraying water to the exhaust gas separated from the fly ash 22 by the separation means 10 and a second scrubber 11b, for spraying an alkali agent-containing water, provided behind the first scrubber 11a. Waste water 25a discharged from the first scrubber 11a is introduced into the mixing means 12, and waste water 25b discharged from the second scrubber 11b is introduced into a part on the downstream side of the mixing means 12.