PURPOSE: To obtain a fibrillar polyaniline having a large surface area and a large aspect ratio by chemical oxidative polymerization oxidizing aniline with a ferric salt or a persulfate in the presence of a specified ion in an acidic aqueous solution.

CONSTITUTION: Aniline (B) is oxidized with an oxidizing agent (C) which is a ferric salt (e.g., ferric perchlorate) or a persulfate (e.g., ammonium persulfate) in the presence of at least one kind of ions (A) selected from among perchlorate, tetrafluoroborate, periodate and tetrafluorophosphate ions in an aqueous solution (D) containing an acid (e.g., perchloric acid) in an amount at least equivalent to that of component B to obtain fibrillar polyaniline of an average diameter ≤1μm, preferably, ≤0.2μm.