The present invention provides a polymer-metal complex composite, which comprises a block copolymer capable of serving as a constituent member of a polymeric micelle and a metal complex having MRI contrast ability, accumulates in a tumor-specific manner, achieves high image contrast even in a small amount, and has reduced side effects and a long retention time in blood. The polymer-metal complex composite of the present invention comprises a block copolymer (A) represented by general formula (a) and a metal complex (B) having MRI contrast ability, wherein the composite comprises a structure in which a carboxyl anion of poly(carbo) in the copolymer (A) is attached to the metal complex (B) via a metal atom (M).
