PURPOSE: To form and recover NaOH and NaCl by a method wherein a harmful exhaust gas containing hydrogen chloride, sulfur oxides and the like is clarified by treating it with an aqueous solution of NaOH and the resulting aqueous solution is electrolyzed.

CONSTITUTION: The exhaust gas 2 containing hydrogen chloride, SOx and the like is brought into reaction with an aqusous NaOH solution in a smoke scurbber to form NaCl and other salts such as Na2SO4 and Na2SO3. Simultaneously, heavy metals, Ca, Mg or the like contained in the exhaust gas are also removed by dissolving them into the solution, and the gas is discharged as a clean gas via an outlet 5. From the aqueous solution thus obtained and containing vairous salts and the like as solutes, ions heavy metals, Ca, Mg, SO4, SO3 and the like are removed by precipiration in a purification plant 7, and the residual solution 8 containing NaC is electrolyzed in a diaphragmless electrolytic cell 9 to reduce the COD value. The electrolyzed solution is fed into a diaphragm-type electrolytic cell 11, where it is further eletrolyzed to produce Cl gas and NaOH. The Cl gas is taken out as a raw material as such or is brought into reaction 16 with NaOH 17 to produce NaClO of high purity, while NaOH 3' is reused by recirculating it to the scrubber 4.