PURPOSE: To efficiently remove O3 contained in exhaust gas by washing the O3- contg. gas with a soln, of below a specific concn. of CaSO3 contg. a sight amt. of an alkali (earth) metal iodide dissolved.

CONSTITUTION: Nox-contg. combustion exhaust gas 1 is introduced into cooling tower 2, cooled to below 90°C, and fed to oxidation tower 3 together with O3 11 to oxidize NO to NO2 and N2O5. The oxidized exhaust gas is then supplied to denitration tower 4 and treated with water or dil. nitric acid to remove the NO2 and N2O5 by absorption. The excess O3-contg. gas leaving tower 4 is fed to residual O3 treatment tower 5. In tower 5 a soln. of below 40 millimol/l of CaSO3 contg. a slight amt. of KI is being circulated through line 6 and pump 7, and the gas is washed with the soln. to remove the excess O3 by absorption. Gypsum formed by the treatment is recovered 12 by centrifugal separation 9 of the washing soln. On the other hand, the filtrate is introduced into washing soln. preparation vessel 13, supplied with CaSO3 in an amt. corresponding to O3 in exhaust gas, and returned to tower 5. The amt, of CaSO3 consumed by the above O3 washing just corresponds with the amt. of O3 contained in the exhaust gas.