To provide an organochlorine pesticide decomposition agent capable of simply decontaminating hexachlorocyclohexane (BHC) as an organochlorine pesticide in a short period and leaving no harmful and hard-to-treat by-products; and a decontamination method using the organochlorine pesticide decomposition agent.

This organochlorine pesticide decomposition agent includes porous copper-containing iron powder formed by mechanically mixing porous iron powder with copper source. Preferably, the porous iron powder includes reduced iron powder, the copper source has a mode including copper salt, metal copper or copper solution and the metal mixture has a mode executed by using a milling-mixing device of a type actuating milling medium in a container such as a vibratory ball mill, a rotary ball mill or the like.