A DDS preparation of a platinum complex, which selectively releases a highly active platinum complex in cells that are under reducing conditions, and exhibits high antitumor activity that is required from a medicine, is still not available, and there is a demand for a novel DDS preparation of a platinum complex that may be used in clinical fields. There is provided a polymer conjugate of a hexa-coordinated platinum complex, the polymer conjugate comprising a block copolymer having a polyethylene glycol structural moiety and a polyaspartic acid moiety or a polyglutamic acid moiety; and a hexa-coordinated platinum complex having a halogen atom and a hydroxyl group at the axial positions, the hexa-coordinated platinum complex being bonded, directly or via a spacer, to a side-chain carboxyl group of the block copolymer.