PURPOSE: To improve the safety and reliability of the titled apparatus, by supplying a high water-content regenerated exhaust gas discharged from the air supplying means into the circulating cooling water of an ozonizer, and using the obtained Cl--free water as the replenishing water of the cooling means.

CONSTITUTION: The three-way valve 123 is operated by the changeover means 125 by the signal from the liquid level detector 124 of the circulating cooling water tank 118, and the regenerated exhaust gas discharged from the adsorption column 108 is introduced into the water tank 118. The water-content of the introduced exhaust gas is lowered to the saturated vapor pressure corresponding to the temperature of the cooling water in the tank 118, and the tank is supplied with cooling water by the lowering of the water-content of the gas. When the water level in the tank 18 is raised to the preset level by the replenishment, the three-way valve 123 is operated by the means 125 by the signal outputted from the detector 124, and the exhaust gas is discharged to the atmosphere. If the temperature of the regenerated exhaust gas is increased to the preset level before the water level in the tank reaches the preset water level, the exhaust gas from the adsorption column 108 is discharged to the atmosphere, and the replenishment is carried out by using the regenerated exhaust gas of the adsorption column 107.