To provide a decomposer which can simply decompose organohalogen compounds, especially hard-to-decompose BHC contained in soil or water within a short term without leaving harmful difficult-to-treat by-products, to provide a method for producing the decomposer, and to provide a method for purifying soil or water containing organohalogen compounds such as BHC.

The decomposer for organohalogen compounds is obtained by adding a copper solution to an iron powder in an agitated state to replace the copper ions in the solution with the iron powder to deposit copper on part of the surfaces of the particles of the iron powder. The decomposer comprises a copper-containing iron powder constituted of particles being iron powder particles and copper present on part of each of them and desirably has a copper content of 0.1-20 mass%, an average particle diameter of 0.1-500 μm, and a specific surface area of 0.1-30 m2/g. The decomposer is added to soil or water containing organohalogen compounds to decompose the organohalogen compounds.