PROBLEM TO BE SOLVED: To provide a substance specifically binding to brain tumor cells which can be used as a guiding substance or the like in DDS for targeting brain tumor and can be efficiently produced compared to an antibody or the like.SOLUTION: The substance is a dimer nonnatural peptide having any of nonnatural amino acid sequences of Bph-2Np (1) Bph-Bph (2) Bph-Aib (3) and Bph-Thi (4) [wherein Bph is β-(4-biphenyl)-alanine 2Np is N-β-(2-naphthyl)-alanine Aib is α-aminoisobutyric acid and Thi is N-β-(2-thienyl)-alanine]. Preferably, the substance is a trimer nonnatural peptide having a nonnatural amino acid sequence of Bph-2Np-Sar (5) or Bph-2Np-Amb (6) [wherein Bph is β-(4-biphenyl)-alanine 2Np is N-β-(2-naphthyl)-alanine Sar is sarcosine and Amb is p-aminomethylbenzoic acid].
