PURPOSE: To remove impurities completely and improve the ozone generating efficiency, by means of a discharge type gas reactor provided between a gas cooler and a gas dryer in an ozonizer by the oxygen recycle.

CONSTITUTION: Oxygen gas is fed to an ozonizer 1, partially ozonized and then fed to an ozone reactor 2, e.g. for bleaching pulp, and consumed for the bleaching action. Oxygen is refed to the ozonizer 1 by a blower 3. In this case, the oxygen gas discharged from the ozone reactor 2 contains water volatile organic substances, which are cooled in a condenser 4 and removed as a drain. The oxygen gas is cooled by a cooler 5 to remove water and volatile organic substances which cannot be removed in the condenser 4. The remaining organic substances are decomposed into CO2 and H2O in a discharge reactor 7 and then removed in the next change-over gas dryer 6 completely. The oxygen gas is then fed to the ozonizer 1 to improve the ozone generating efficiency.