A peptide that can be used as an imaging probe for GLP-1R is provided. In an embodiment, a polypeptide is represented by the following formula (3)；where Xaa1 represents an aspartic acid in which a -Y-X' group binds to an α-amino group, X' includes a chelating site and a radioactive metal nuclide chelated by the chelating site, the chelating site being diethylenetriaminepentaacetic dianhydride (DTPA) or 1,4,7-triazacyclononnane-N,N',N"-triacetic acid (NOTA), and Y represents a linker including a group selected from the group consisting of -CH2-(C6H4)-, -NH-C(=S)-, -NH-(CH2) 5-C(=0)-, and a combination thereof.
