PURPOSE: To accomplish efficent decomposition to separate thiosulfate contained in the liquid by limiting the temperature and pH of the liquid in an acid decomposition of a thiosulfate-contained liquid while SO2 generated is converted into sulfuric acid to be recycled for the acid decomposition.

CONSTITUTION: A thiosulfate-contained liquid withdrawn from an absorption column 1 is fed to an acid decomposition tank 5 from a line 21 where the thiosulfate is decomposed by sulfuric acid circulated from a line 22 with the liquid controlled to 40 to 90°C in temperatur and 0.5 to 3 in pH. SO2 herein generated is put together with air and subjected to dehumidification in a drying column 4 to remove mist. Then, it fed to a converter 3 where SO2 is converted to SO3 by a oxidizing catalyst. SO3 thus obtained is fed to a SO3 absorption column 2 where it is absorbed by sulfuric acid circulated through a cooling device 10 to be converted to sulfuric acid. The sulfuric acid is recycled through the line 22. With such an arrangement, SO2, a polutant, can be converted to sulfuric acid necessary for the acid decomposition without leaving the system. In this manner, the thiosulfate can be decomposed away effectively