PURPOSE: To accelerate decomposition reaction and to use an oxidizing agent to be injected efficiently by decomposing org. materials by oxidation with ozone alone in an initial period of catalytic reaction and adding and using hydrogen peroxide in combination to and with the ozone in the 2nd reaction region where the reaction retards.

CONSTITUTION: Org. materials such as humic acids in water are decomposed by oxidation with a treatment by the ozone alone in the 1st reaction region in an initial period of catalytic reaction. In the 2nd reaction region where reaction retards, hydrogen peroxide is added and used to and in combination with the ozone to accelerate the decomposition reaction. The two-stage treating method making combination use of ozone and hydrogen peroxide is effective as a method for removing humic acid as a precursor for formation of trihalomethane and prevents the formation of trihalomethane in both terms of an effet of decreasing TOC components and chemical stability of residual TOC components.