PURPOSE: To produce iron powder having improved corrosion resistance and weatherability by packing the vapor of a chlorine based org. solvent and steam into a treating rank sealed with the iron powder, eluting the deposits in the gaps of the iron powder structures and forming rust of high corrosion preventiveness on the iron powder surfaces.

CONSTITUTION: Heating and cooling pipes 20 are arranged in the lower part in the treating tank and the iron powder W is placed on a placing stage 26 disposed above these pipes. A liquid mixture composed of water Wa, a surfactant liquid S and a methylene chloride soln. Me is injected into the tank in such a manner that the iron powder W is not soaked. The steam heated to about 70 to 150°C is then supplied to the pipes 20 to evaporate the water Wa, the surfactant liquid S and the methylene chloride soln. Me. The respective vapors are infiltrated into the gaps of the structures of the iron powder W to elute the deposits in the gaps and to form passive films consisting of the rust (Fe3O4) having the high corrosion preventiveness on the surfaces of the iron powder W. The gaseous Cl2 generated from the methylene chloride Me at this time is absorbed in the water Wa to suppress the adverse influence thereof on the treatment.