To efficiently and economically decompose a volatile organic halogen compound by heating a gaseous mixture through plural stages from a gas adjusting process to a catalytic decomposition process and controlling the gaseous mixture to a specific max. temp. directly before the contact with a decomposition catalyst.

In the case of treating the volatile organic halogen compound in a waste water 6, the waste water 6 and air 8 are supplied to a stripping device 1 to produce the volatile organic halogen compound-containing gas 9. Next, the gas is sucked into an adsorption device 2 through a pipe line 12 by a pump 7. A regenerating gas 17 obtained by heating air by a heater 16 is introduced into the adsorption device 2 and the volatile organic halogen compound is stripped from an adsorbent. The resultant volatile organic halogen compound-containing regenerating gas 19 after steam 20 is added is heated by a preliminary heating device 3, introduced into a catalytic reaction device 4 and is decomposed by the contact with a catalyst layer 21 after heated to reach 110-130°C by a heater 22.