PURPOSE: To enhance treating efficiency and to prevent secondary pollution due to heavy metals, by electrolyzing aqueous NaCl solution with a predetermined concn. obtained by treating a waste gas with NaOH at a predetermined decomposition ratio in an ion exchange membrane electrolytic tank to be recirculated.

CONSTITUTION: A waste gas containing HCl is sent into a gas washing apparatus 2 to be reacted with an aqueous NaOH solution and an aqueous NaCl solution with a concn. of 20wt% or more is formed. In the next step, the aqueous NaCl solution taken out of the apparatus 2 is introduced into a purification process 3 to remove heavy metals and the treated solution is supplied to the anode chamber 6 of an ion exchange membrane electrolytic tank 5. In this electrolytic tank 5, said treated liquid is electrolyzed at a decomposition ratio of 40W60% and the electrolyte in the chamber is supplied to the cathode chamber 7 thereof to recirculate the aqueous NaOH solution from the chamber 7 to the apparatus 2.