Provided is a cyclic peptide that is represented by formula (I) or formula (I), has excellent antibody-binding characteristics, and also has improved drug stability. Also provided are an affinity chromatography support, a labeled antibody, an antibody-drug conjugate, and a pharmaceutical preparation. RN-Xg-[Xi-Xa-Xm-X1-X2-X3-Xn-Xb-Xj]k-Xh-RC...(I) In formula (I): Xa and Xb each independently represents an amino acid residue derived from an amino acid that has a thiol group in a side chain thereof other than L-cysteine and D-cysteine, Xa and Xb each being bonded via a disulfide bond or one of Xa and Xb represents an amino acid residue derived from an amino acid that has a thiol group in a side chain thereof other than L-cysteine and D-cysteine and the other represents an amino acid residue derived from an amino acid that has a haloacetyl group in a side chain thereof, Xa and Xb each being bonded via a thioether bond. RN-Xg-[Xi-Xa-Xm-X1-X2-X3-Xn-Xb-Xj]k-Xh-RC...(I) In formula (I): one of Xa and Xb represents an amino acid residue derived from L-cysteine or D-cysteine and the other represents an amino acid residue derived from an amino acid that has a haloacetyl group in a side chain thereof, Xa and Xb each being bonded via a thioether bond or one of Xa and Xb represents an amino acid residue derived from L-penicillamine or D-penicillamine and the other represents an amino acid residue derived from an amino acid that has a haloacetyl group in a side chain thereof, Xa and Xb each being bonded via a thioether bond.Linvention concerne un peptide cyclique qui est représenté par la formule (I) ou la formule (I), présente dexcellentes caractéristiques de liaison danticorps et présente également une meilleure stabilité de médicament. Linvention concerne également un support de chromatographie daffinité, un anticorps marqué, un conjugué anticorps-médicament et une préparation pharmaceutique. RN-Xg-[Xi-Xa-Xm-X1-X2-X3-Xn-Xb-Xj]k-Xh-RC...(I) Dans la formule (I) : Xa et Xb représe
