PROBLEM TO BE SOLVED: To provide a novel compound which has excellent A&bgr； aggregation inhibitory activity and is useful as a pharmaceutical product.SOLUTION: A cyclic peptide or salt thereof comprises an amino acid sequence represented by formula (1) of X-Leu-Val-Y-Y. In the amino sequence, an &alpha；-amino group at the amino terminal and a carboxyl group at the carboxyl terminal are linked with each other by a peptide bond. In the formula (1), X represents Lys, Arg, His, Ala, Gly, Ser or Thr, and Yand Ymay be the same or different and each represent a group represented by formula (2). In the formula (2), Arrepresents an aromatic hydrocarbon group or an aromatic heterocyclic group, where the aromatic hydrocarbon group and the aromatic heterocyclic group may have one to five substituents selected from an alkyl group, a cycloalkyl group, a haloalkyl group, a halogen atom, a hydroxy group, an alkoxy group, an aromatic hydrocarbon group, an aromatic heterocyclic group and an amino group.【課題】優れたＡβ凝集阻害作用を有し、医薬として有用な新たな化合物の提供。【解決手段】次式（１）で表されるアミノ酸配列からなり、該アミノ酸配列のアミノ末端のα－アミノ基とカルボキシル末端のカルボキシル基がペプチド結合で連結された環状ペプチド又はその塩。X-Leu-Val-Y1-Y2（１）（式（１）中、ＸはLys、Arg、His、Ala、Gly、Ser又はThrを示し、Ｙ1及びＹ2は同一又は異なって、式（２）で表わされる。）（式（２）中、Ａｒ1は、芳香族炭化水素基又は芳香族複素環式基（ここで、芳香族炭化水素基及び芳香族複素環式基は、アルキル基、シクロアルキル基、ハロアルキル基、ハロゲン原子、ヒドロキシ基、アルコキシ基、芳香族炭化水素基、芳香族複素環式基及びアミノ基から選ばれる１～５個の置換基を有していてもよい）を示す。）【選択図】なし
