PURPOSE: To produce a metallic fiber for a filter having a larger diameter than a conventional fiber by oxidizing the surface of a metallic fiber at a high temp., forming oxide whiskers on the surface of the resulting metallic oxide, and reducing the oxide whiskers.

CONSTITUTION: For example, an iron fiber is oxidized at a high temp. in a gaseous Ar-O2 mixture to form FeO on the surface of the fiber to about 5μm average thickness. The gaseous Ar-O2 mixture is mixed with gaseous H2, H2O and N2 until the partial pressure of oxygen is regulated to 10-14 atm. under the same temperature, and the iron fiber is exposed in the resulting gaseous mixture to form FeO whiskers on the surface of the oxide. The surface of the fiber and the FeO whiskers are then reduced in gaseous H2.