PURPOSE: To pressurize a highly concentrated ozone-containing gas under high pressure by blending an ozone-containing gas with a liquid, pressurizing the gas-mixed liquid and separating the liquid into the ozone-containing gas and liquid by a gas separating tank.

CONSTITUTION: First, an ozone-containing gas generated by an ozonizer 1 is mixed with a liquid by a mixer 2, etc., the gas-mixed liquid is pressurized by a pump 3. Then, the pressurized gas is introduced into a gas separating tank 4 and separated into the ozone-containing gas and the liquid to give a pressurized ozone-containing gas. Preferably, the liquid remaining in the tank 4 is evacuated by a pressure reducing device 5 to a pressure ≤delivery pressure of the ozone-containing gas by the ozonizer 1 and reused.