An electroconductive organic polymer exhibiting outstanding electroconductivity is described, containing an electron acceptor as a dopant, and having electroconductivity of not less than 10.sup.-6 S/cm, substantially comprising a linear polymer having as a main repeating unit thereof a quinonediimine structure represented by formula (III) ##STR1## wherein R represents a hydrogen atom or an alkyl group. This polymer can be obtained by oxidative polymerization of an aniline compound or a water-soluble salt thereof with an oxidizing agent in a reaction medium containing a protonic acid.