The present invention provides: a block copolymer which has an improved effect of imaging the inside of a tumor compared to known block copolymers and imaging probes using the known block copolymers； and an imaging probe using said block copolymer. Specifically, compared to known block copolymers and imaging probes using the known block copolymers, in order to efficiently improve an imaging effect by enhancing permeability to target diseased tissue, provided are: 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, on a side chain thereof, a signal group and an arbitrarily defined hydrophobic substituent, wherein the average molecular weight of a main-chain polymer which is the combination between the polyethylene glycol chain and the polyamino acid chain is 2-10 kDa, the content by mass of the signal group and the arbitrarily defined hydrophobic substituent is 5-50 mass%； and a composition containing the block copolymer.本發明提供一種較公知之嵌段共聚物、及使用該嵌段共聚物之成像探針而言，腫瘤內部之成像效果有所提昇之嵌段共聚物，及使用該嵌段共聚物之成像探針。具體而言，以使向疾病靶組織之浸透性較公知之嵌段共聚物、及使用該嵌段共聚物之成像探針提昇從而有效率地提昇成像效果為目的，本發明係一種嵌段共聚物、及包含該等嵌段共聚物之組合物，該嵌段共聚物係含有聚乙二醇鏈之親水性聚合物鏈段與含有側鏈具有訊號基及任意疏水性取代基之聚胺基酸鏈的疏水性聚合物鏈段連結而成者，且該聚乙二醇鏈與該聚胺基酸鏈合併之主鏈聚合物之平均分子量為2 kDa以上且10 kDa以下，訊號基及任意疏水性取代基之質量含有率為5質量%以上且50質量%以下。
