PURPOSE: To efficiently oxidize a sulfur compd. by allowing an aq. medium containing a low valent sulfur compd. to which hydrogen peroxide is mixed to contact with a solid catalyst containing a specified metal and having ≥10m2/g surface area.

CONSTITUTION: The solid catalyst is formed by depositing a transition metal selected from the group consisting of vanadium, chromium, molybdenum, tungsten, copper, nickel, manganese, cobalt and iron on a metallic oxide base material selected from the group consisting of aluminum oxide and silicon oxide and their mixture, preferably aluminosilicate, and having at least 10m2/g surface area. The low valent sulfur compd. is oxidized by allowing the aq. medium containing the low valent sulfur compd. to contact with the solid catalyst. Fulfide, etc., are exemplified as the low valent sulfur compd..