[Problem] A DDS preparation of a platinum complex showing the high antitumor effect and reduced side effects required as a medicine, that is, a clinically useable DDS preparation of a platinum complex characterized by being conjugated to a polymer carrier different from conventional ones, is desired. [Solution] Provided is a polymer conjugate of a platinum(II) complex, in which the sulfoxide group of a sulfoxide derivative introduced into a side chain carboxy group in a block copolymer having a polyethylene glycol structural moiety and a polyaspartic acid moiety or a polyglutamic acid moiety is coordinated to the platinum.
