PROBLEM TO BE SOLVED: To provide a polymer with a high tumor blood ratio as a contrast medium for photoacoustic imaging.SOLUTION: The present invention provides a polymer represented by formula (P1) (R is a residue or a functional group derived from a polymerization initiator； A may not exist and represents, when it exists, a low molecular weight compound, a dye, a reporter molecule, a target bonding molecule, a polymer, or D； L is a linker, where L may not exist； nis an integer of 1 or more； D is a dye skeleton of a dye having absorption in the near-infrared region).SELECTED DRAWING: Figure 1【課題】腫瘍血液比が高い重合体を光音響イメージング用造影剤として提供する。【解決手段】式（Ｐ１）で表わされる重合体。（Ｒは重合開始剤に由来する残基又は官能基；Ａは存在しなくとも良く、存在する場合は低分子化合物、色素、レポーター分子、標的結合性分子、重合体、又はＤ；Ｌはリンカーで、Ｌは存在しなくてもよい；ｎ１は１以上の整数；Ｄは近赤外領域に吸収を持つ色素の色素骨格）【選択図】図１
