To obtain the subject thin membrane capable of forming into a very small shape and easily controlling the thin membrane by containing an organic magnetic particles containing an organic magnetic material such as a tetraazaporphyrin derivative hardly soluble to a solvent in a polymer thin membrane obtained by polymerizing a monomer and/or an oligomer containing the same.

This organic magnetic membrane (C) is obtained by electrolytically polymerizing (B) a monomer and/or an oligomer in the presence of (A) an organic magnetic material and containing the particles of the component (A) in the thin membrane of the polymer of the component (B). Here, the particles of the component (A) contains at least the organic magnetic material of a tetraazaporphyrin derivative of formula I (M is a magnetic metal or a compound selected from a group of the magnetic metal compounds; A is an organic residue of formula II, III, etc., two H or bonds) or a porphyrin derivative of formula IV bound with an alkali metal, preferably Na, K or Rb. Further, as the M, at least one element of Fe, Co, Ni and Mn is preferable.