To provide the technology capable of efficiently decomposing a gase ous halogenated aliphat hydrocarbon compound by a convenient device constitu tion and also hardly causing the problem of the secondary polluting due to decomposition product and excellent in environmental protection.

In a method and device for decomposing the gaseous halogenated aliphat hydrocarbon compound, a stage in which the gaseous halogenated aliphat hydrocarbon compound and gaseous chlorine-containing gas are introduced into a reaction region to mix, a stage in which the gaseous halogenated aliphat hydrocarbon compound is decomposed by irradiating the mixed gas with light and a stage in which the gas containing a decomposed product of the gaseous halogenated aliphat hydrocarbon compound is brought into contact with a photocatalyst under photoirradiation to moreover decompose the gas are provided.