PURPOSE: To correctly control the adequate quantity of chlorine by controlling both the blowing of the circulating gas into a solvent and the heating of the solvent with only the pressure in a gas blowing pipe.

CONSTITUTION: When the gas pressure in a gas blowing pipe is decreased, a blow adjustment automatic valve 1 is closed to blow the gas into a chlorine absorbing solvent. When the pressure is further decreased, a heat adjustment automatic valve 2 is opened to heat a solvent, and the generation of chlorine is accelerated. When the gas pressure is increased, the heat adjustment automatic valve is closed to stop the heating of chlorine. When the pressure is further increased, the blow adjustment automatic valve 1 is opened to stop the blowing of the gas. Since the control system is operated by the pressure only, the adequate control can be performed, e.g., the temperature rise speed is changed by the quantity of chlorine in the solvent.