PURPOSE: To precipitate the metal fiber out of the slurry containing ferromagnetic metal fiber by mixing the decomposed product of the zero-valence organic metal complex with specific aspect ratio where the ligand of the ferromagnetic metal is stabilized with the ligand-stabilized zero-valence metal complex.

CONSTITUTION: The zero-valence complex of the metal for the ferromagnetic metal fiber consisting of at least one kind of Fe, Co and Ni, and containing at least one kind of metals of Ti, V, Cr, Cu, Nb, Mo, Ag and Au as the arbitrary metal, is mixed with the organic liquid medium such as toluene and xylene which is non-reactive to the metal fiber to form the slurry. At least one kind of porous or non-porous base material is present in the liquid medium under the presence of the magnetic field externally applied to the slurry, and the energy is applied thereto to decompose the zero-valence metal complex, and the metal fiber of 100:10,000 in aspect ratio is precipitated in the base material so as to be passivated.