An imaging probe comprising a block copolymer comprising a hydrophilic polymer segment linked to a hydrophobic polymer segment, the hydrophilic polymer segment containing a polyethylene glycol chain, and the hydrophobic polymer segment containing a polyamino acid chain having a signal group and optionally a hydrophobic substituent in a side chain, and
- wherein the block copolymer is represented by General Formula (1):
- wherein R1 represents a hydrogen atom or a linear, branched or cyclic (C1-C6) alkyl group which may have a substituent;
- t represents an integer from 20 to 140;
- A represents a (C1-C6) alkylene group which may have a substituent;
- R2 represents a substituent selected from the group consisting of a hydrogen atom, a (C1-C6) acyl group, and a (C1-C6) alkoxycarbonyl group;
- R3 represents a signal group;
- R4 represents a hydrophobic substituent;
- n represents 1 or 2;
- x1, x2, y1, y2, and z each independently represents an integer from 0 to 20;
- (x1+x2) represents an integer from 1 to 10;
- (x1+x2+y1+y2+z) represents an integer from 3 to 20;
- and the various constituent units to which R3 and R4 are bonded, and a constituent unit formed by intramolecular cyclization of a side-chain carbonyl group constitute a structure with those constituent units being each independently randomly arranged;
- wherein said signal group of R3 contains a fluorescent group, a radioisotope-containing group, or a magnetic group, said fluorescent group including a group derived from a member selected from the group consisting of a fluorescein-based dye, an indocyanine-based dye, a rhodamine-based dye, a BODIPY-based dye, a xanthene-based dye, a Nile Red-based dye and a quantum dot, said radioisotope-containing group including a group derived from a member selected from the group consisting of a sugar, an amino acid and a nucleic acid, all being labeled with radioisotopes, said magnetic group including a group selected from the group consisting of ferrichrome, ferrite nanoparticles and magnetic nanoparticles;
- wherein the main chain polymer of the block copolymer 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, said average molecular weight of the main chain polymer being a calculated value obtained by summing the (1) average molecular weight of the polyethylene glycol chain and the (2) average molecular weight of the polymer main chain portion of the polyamino acid chain excluding the signal group and the optional hydrophobic substituent, said average molecular weight of the polyethylene glycol chain being determined from the peak top molecular weight measured by a GPC method based on polyethylene glycol standards, and average molecular weight of the polymer main chain portion of the polyamino acid chain being a calculated value obtained by multiplying the average molecular weight of the polymerized monomer unit of the polyamino acid chain by the average number of units of the polyamino acid obtained by neutralization titration method, and a 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
- wherein the block copolymer has a particle size of less than 20 nm.