To solve the problem that only substances permitted to emit to the atmosphere such as CO2, H2O, N2and O2are by-products, a catalytic combustion method cannot be used because of catalyst degrading in a system which generates useful hydrogen chloride at the same time by a high-temperature decomposition treatment, and that the recovery of energy used for the high-temperature treatment together with the generated hydrogen chloride cannot be performed even if the system may be decomposed without catalysts.

In order to recover heat generated in a combustion furnace and organic constituents contained in a process exhaust gas while a by-produced hydrogen chloride from Cl contained in a process content gas is recovered, heat is firstly recovered using a heat recovering agent which absorbs no hydrogen chloride such as an aqueous solution of sulfuric acid, and thereafter the by-produced hydrogen chloride is recovered by a hydrogen chloride recovering agent such as water. Thus, the gas containing a chlorinated organic compound and N2O can be treated efficiently and at a low cost.

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