PURPOSE: To obtain a cured material without causing the peeling of formed coating layer and the inclusion of impurity gas in the material by applying, impregnating or attaching a coating layer of an organic compound to a formed carbon fiber article to close the pore of the formed article and thermally curing the coating layer from the inside by Joule heat.

CONSTITUTION: A formed carbon fiber article 1 used as an anode is immersed in an electrolyte solution containing a supporting electrolyte and an organic compound monomer (e.g., alcohol, aromatic hydrocarbon, polycyclic aromatic hydrocarbon, glycol, sugar or tar) and electric current is passed through the solution to effect the electrolytic polymerization of the monomer on the fiber surface and form a uniform coating layer of the polymer in the crevice (e.g., pore), gap and outer surface of the formed article. The formed article 1 is slowly pulled up while passing the electric current through the solution and finally taken out of the electrolyte solution. The formed article 1 is heated from inside by the Joule heat generated at the exposed part 4 to start the curing of the polymer coating layer from the inner surface at the interface between the layer and the substrate. The curing successively proceeds while expelling the impurity gas from the coating layer to obtain a uniformly cured article.