PURPOSE: To enhance ozone dissolving efficiency, to dispense with the treatment of waste ozone and to enhance a COD removing rate.

CONSTITUTION: A packed bed 2 of granular activated carbon 15 at least partially immersed in a liquid is provided to a column 7 and raw water 8 to be treated is supplied to the column 7 as a descending or ascending current from an inflow water pipe 5 or 11 while ozone gas 9 is directly diffused to the packed bed 2 from the ozone diffuser 3 arranged to the lower part of the packed bed 2 and passed through the packed bed 2 to oxidize and remove org. matter such as COD or chromaticity in the raw water by ozone while clear treated water is obtained from a water discharge pipe 6.