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), R0 represents any of a residue or a functional group derived from a cationic polymerization initiator; L represents a linker and L may not be present; n represents an integer of 1 or more; D represents a dye backbone of a dye having absorption in the near-infrared region.