Abstract: Waste gas containing sulfur oxide (SO.sub.x) and nitrogen oxide (NO.sub.x) is mixed with an ozone-containing gas to have the value x of NO.sub.x fall in the range of 1.2 to 2.5 and thereafter brought into contact with an aqueous solution containing sulfuric acid, nitric acid and an iron compound to be deprived of SO.sub.x and NO.sub.x. To the solution with which the waste gas has been brought into contact, air or oxygen is added to effect oxidation of the part of said oxides still remaining therein in an unoxidized state and regenerate the activity of the iron compound functioning as a catalyst. Part of the solution to which the addition of air or oxygen has been made is recycled as the solution for the purpose of contact with waste gas.