A drug delivery system (DDS) preparation of a platinum complex which selectively releases a highly active platinum complex in cells in reductive conditions, and exhibits the required high antitumour activity as a drug is yet to be obtained, and thus a novel DDS preparation of a platinum complex which can be used clinically is in demand. Accordingly, a polymer conjugate of a hexa-coordinated platinum complex is provided in which the hexa-coordinated platinum complex, which has halogens and hydroxyl groups at axial positions, is bonded directly or via spacers to side-chain carboxyl groups of a block copolymer having a polyethylene glycol structure moiety and a polyaspartic acid moiety or a polyglutamic acid moiety.
