Provided are a block copolymer having an enhanced effect of imaging the interior of a tumor compared to known block copolymers and imaging probes using the known block copolymers； and an imaging probe using the block copolymer. Specifically, for the purpose of enhancing penetrability into a diseased target tissue and efficiently enhancing an imaging effect compared to known block copolymers and imaging probes using the known block copolymers, disclosed is a block copolymer in which a hydrophilic polymer segment containing a polyethylene glycol chain is linked to a hydrophobic polymer segment containing a polyamino acid chain having a signal group and optionally a hydrophobic substituent in a side chain, wherein the main chain polymer combining the polyethylene glycol chain and the polyamino acid chain has an average molecular weight of not less than 2 kDa and not more than 10 kDa, and the mass content percentage of the signal group and the optional hydrophobic substituent is not less than 5% by mass and not more than 50% by mass； and also disclosed is a composition including the block copolymer.
