PURPOSE: To oxidize sulfuric compounds existent in waste water into sulfuric acid, by bringing a filler obtd. by mixing the predetermined amount of sulfur or a sulfide in the salt of a sulfuric oxyacid and hydrothermally treating the reaction product under pressure into contact with waste water containing dithionic and polythionic acids under an aerobic condition.

CONSTITUTION: The predetermined amount of sulfur or a sulfide is mixed in the salt (excluding an ammonium salt) of a sulfuric oxyacid, a caking accelerator is added at need, and the mixture is formed. The formed mixture is hydrothermally treated under pressure to obtain a filler. This filler is brought into contact with waste water containing sulfuric compounds based on dithionic and polythionic acids under an aerobic condition, to biooxidize said sulfuric compounds into sulfuric acid. Said salt of a sulfuric oxyacid may be sulfite, thiosulfate, etc., while said sulfide may be ferric sulfide, ferric bisulfide, etc. The mixing ratio (wt. ratio) of the salt of a sulfuric oxyacid to sulfur or a sulfide is pref. about 1:10W100.