Abstract: An oxidizing agent is added to water polluted with organic compounds, especially organic chlorine compounds, disinfection is carried out, and suspended solids in the water are removed. Thereafter, ultraviolet rays are irradiated to oxidize and decompose the organic compounds, and furthermore, volatile organic compounds are separated and removed from the water. Finally, the oxidizing agents residual in the water are reduced. The oxidizing agent is separately added at two phases of disinfection and oxidization decomposition, thereby causing the decomposing efficiency of organic compounds to be much improved.