PURPOSE: To discharge a waste photographic processing solution to rivers by electrolytically treating an aqueous solution containing a thiosulfate ion in a three-dimensional electrode electrolytic cell and oxidizing the thiosulfate ion to a high order sulfur oxide ion.

CONSTITUTION: An aqueous thiosulfuric acid solution is supplied to a three-dimensional electrode-electrolytic cell 2 to be subjected to oxidation treatment. The flow rate of the aqueous thiosulfuric acid solution supplied to the electrolytic cell 2 may be to prescribed so that the aqueous solution efficiently comes into contact with the surfaces of electrodes to oxidize a thiosulfate ion and, when the aqueous solution flows in a perfectly lamina state, the movement of the aqueous solution to a lateral direction is reduced and the contact with the electrodes, dielectrics and the surfaces of fine particles is reduced and, therefore, a turblent flow state is pref. formed. Further, the electrode potential at this time is pref. set to -0.1-1.0vs. SHE in anode potential. The thiosulfate ion is almost selectively oxidized to a polythosulfate ion within this potential range and can be subjected to oxidative decomposition without precipitating sulfur. By this method, a waste photographic processing solution can be discharged to rivers.