To provide a method for decomposing a pollutant within a short time and at a normal temp. for underground water and soil contaminated with an org. chlorine compd.

Soil or underground water is rapidly cleaned by decomposing an org. chlorine compd. by adding and mixing copper-containing iron powder to soil and underground water contaminated with the org. chlorine compd. A copper content of the copper-containing iron powder is 0.1-20 wt.% and addition of the copper-containing iron powder is within a range of 1-10 wt.% per the soil and the underground water. All of 16 kinds of tested org. chlorine compounds exhibit significant decomposition within 13 days and decomposition speed is remarkably improved in comparison with a case where iron powder is used. A decomposition product accompanying reaction is harmless gas such as methane, ethane, ethylene, propane and propene.