PURPOSE: To enable multicolor display of an electrochromic display device by using a specified polyaniline.

CONSTITUTION: A polyaniline coating film obtd. by polymn. of substd. aniline expressed by formula is formed on a substrate. In formula, X1, X2, X3 and X4 are same or different hydrogen atoms or substituents having -0.10 to +0.60 Hamett's value δm+ and at least one of these is not hydrogen atom but the substituent. This polyaniline can be produced by an electrolytic polymn. method by electrolytic oxidation in a soln. containing substd. aniline and an electrolyte or by a chemical polymn. method using an oxidant. In the electrolytic polymn. method, the coating film can be formed directly on the substrate. In the chemical polymn. method, polyaniline is made soluble by dedoping or the like and the soln. is applied and dried on a substrate to form a film.