To completely decompose and separate organohalogen compounds to prevent air pollution by subjecting organohalogen compound containing gas to be treated to gas discharge in the presence of steam of a specified volume ratio or less and at specified concentration or less of oxygen.

A discharge device 2 is connected to a reduced pressure deaeration tank 1, and further a vacuum pump 3 is connected to the discharge device 2, and a suitable pressure adjusting devices, an air control valve 6, a control valves 7, 8 are fitted. Organohalogen compound containing water to be treated is first sent to the deaeration tank 1, which is evacuated to vaporize organohalogen compounds and water from the water to be treated and such them into the discharge device 2. At that time, gas discharge is performed in the presence of steam of 80vol% or less and at oxygen concentration of 10vol% or less. Then OH radicals are generated by the gas discharge, and by strong oxidizing force of the OH radicals, the organohalogen compounds are decomposed.