The present invention provides a polymeric agent which can incorporate a large amount of a drug, accumulate selectively in a tumor site, and has a molecular size of more than that for renal excretion. A metalloporphyrin derivative (such as zinc protoporphyrin) is associated with a styrene-maleic acid copolymer via non-covalent bond to give a SMA micelle complex, allowing provision of a polymeric pharmaceutical agent for treatment of cancer with a large amount of the drug incorporated. The SMA micelle complex can be produced by a method, wherein the metalloporphyrin derivative reacts with the styrene-maleic acid copolymer in the absence of a condensation agent under an alkaline condition, solubilized, adjusted to have a pH of 6-8, and subjected to a procedure for separating a polymer component to recover the micelle complex component for the polymeric pharmaceutical agent.