The present invention provides a new redox-active polymer capable of an adequate redox reaction even at low temperature and effectively usable as a high-capacity, high-energy density battery, a large-area electrochromic element, or a biochemical sensor using a microelectrode. This redox-active polymer is of being prepared by polymerizing an aromatic or heterocyclic compound having two or more thiourea groups with an aromatic or heterocyclic compound having two or more isothiocyanate groups. For example, The redox-active polymer may be of being prepared by polymerizing N,N'-1,4-phenylene-bis-thiourea with phenylene-1,4-diisothiocyanate. The redox-active polymer is suitable as an electrode material, particularly a cathode for lithium secondary batteries.