Disclosed is a redox-complementary electrochromic device exhibiting black-to-transmissive switching, wherein the device comprises an electrochromic layer and a redox-active material layer sandwiched between a transparent first electrode and a transparent secondary electrode, the electrochromic layer comprising an electrochromic Co-based metallo-supramolecular polymer represented by the formula (I), and the redox active material being capable of reacting with the electrochromic material to change the electrochromic material from black state into colorless transmissive state, where in the formula (I), X represents a counter anion, R represents a single bond or a spacer comprising a carbon atom and a hydrogen atom, each of R1 to R4 independently represents a hydrogen atom or a substituent group, and n represents an integer of from 2 to 5000, which indicates a degree of polymerization.