To surely perform harmlessness treatment of a contaminated substance, and to reduce treatment cost of the contaminated substance, when removing organic halogen compounds from a treating object contaminated with organic halogen compounds.

This treating method of the contaminated substance comprises a reducing/heating process heating the contaminated substances contaminated with organic halogen compounds in a reducing atmosphere. Air is separated into high concentration nitrogen and high concentration oxygen, the high concentration nitrogen is supplied to the reducing/heating process, and the high concentration oxygen is used for combustion of a combustion apparatus used for the reducing/heating process.