There is provided a lysine oligomer derivative, wherein an ε-amino group and a carboxyl group of lysines are linked via a peptide bond, and a group capable of generating or absorbing electromagnetic wave is bonded to a C-terminal carboxyl group, an N-terminal amino group and/or an α-amino group. This lysine oligomer derivative has the characteristic of specifically accumulating in the cartilage matrix and can generate or absorb an electromagnetic wave, and is, therefore, useful as a cartilage tissue marker.