The present invention provides a polymer-metal complex composite, whichcomprises a block copolymer capable of serving as a constituent member of apolymericmicelle and a metal complex having MRI contrast ability, accumulates in atumor--specificmanner, achieves high image contrast even in a small amount, and has reducedside effects and a long retention time in blood.The polymer-metal complex composite of the present invention comprises ablock copolymer (A) represented by general formula (a) and a metal complex (B)having MRI contrast ability, wherein the composite comprises a structure inwhich acarboxyl anion of poly(carbo) in the copolymer (A) is attached to the metalcomplex (B)via a metal atom (M).
