To provide a decomposition treatment method for halogenated organic matter in which treated water surely satisfies a standard of water for completing a cource and to provide an apparatus.

A polyviphenyl chloride 36 is dehalogenated by an apparatus 6 for irradiation with UV rays and is further dehalogenated by a biological treatment apparatus 8 and is then introduced into a hexane cleaning apparatus 10. The treated liquid 46 satisfying the graduation standard is released into sewer from the cleaning apparatus 10. The slightly remaining polyviphenyl chloride 44 is made confluent with the halogenated organic matter 36 and is again dehalogenated by the apparatus 6 for irradiation with UV rays.