PURPOSE: To absorb and remove sulfur dioxide in waste gas with an extremely high efficiency, by contacting waste gas with water solution containing ferrous ion-chelate complex or moreover, ferric ion-chelate complex obtained by oxidizing ferrous ion- chelate complex.

CONSTITUTION: SO2 is absorbed and removed by contacting waste gas containing SO2 and oxygen, with absorbing liquid containing ferrous ion-chelate complex or moreover, ferric ion-chelate complex obtained by oxidizing ferrous ion-chelate complex at absorbing tower. Next, the absorbing liquid after contacting with the waste gas, is fed to the electrolysis vessel and ferric ion-chlate complex generated in the absorbing tower and reaction product by SO2, are electrolytically reduced and then, ferrous ion-chelate complex is regenerated. The absorbing liquid is recirculated to the absorbing tower again and is used semipermanently, after heating reduced absorbing liquid and recovering SO2 as concentrated gas.