PURPOSE: To improve liquid-gas contact effect by dividing the inside of an upright vessel to three or more small reaction chambers by means of two or more sheets of horizontal porous plates having plural pieces of holes and continuously supplying the water to be treated and ozone from the small reaction chamber at the bottom end.

CONSTITUTION: The water to be treated is continuously supplied into the small reaction chamber 9 at the bottom end of an upright cylindrical vessel 8, and a gaseous ozone is supplied from an ozone gas generator 15 into an air diffusion pipe 10 for ozone injection in the small reaction chamber 9 at the bottom end. The bubbles of the gaseous ozone and the water to be treated migrate successively to the small reaction chambers in the upper parts in the parallel gas-liquid flow, and at this time, ozone is rediffused by the small holes 16 of the respective porous plates 17, whereby small bubbles are produced and the good gas-liquid contact state is assured. From the small reaction chamber 11 at the top end, the ozone treated water flows into a final treating tank through a piping 12, and the flow consisting essentially of the gas contg. unreacted ozone flows through a piping 13 and is released into the atmosphere through an active carbon tower 14.