An object is to provide a polymer having a high tumor/blood ratio. The present invention provides a polymer represented by the following formula (P1):wherein in the formula (P1), R represents any of a residue derived from a polymerization initiator, or a functional group； A may not be present, and when present, A represents any of a low-molecular compound, a dye, a reporter molecule, a target-binding molecule, a polymer or D； L represents a linker and L may not be present； n1 represents an integer of 1 or more； and D represents a dye backbone of a dye having absorption in the near-infrared region.
