A DDS preparation of a platinum complex, which exhibits a superior antitumor effect required as a medicine and reduced side effects, that is, a clinically usable DDS preparation of a platinum complex, which is conjugated to a polymer carrier that is different from conventional carriers, is desired. Provided is a polymer conjugate of a platinum(II) complex, the polymer conjugate including: a block copolymer having a polyethylene glycol structural moiety and a polyaspartic acid moiety or a polyglutamic acid moiety； a sulfoxide derivative introduced into a side-chain carboxyl group of the block copolymer； and a platinum(II) complex coordinate-bonded to a sulfoxide group of the sulfoxide derivative.
