The present invention relates to a polypeptide represented by the formula: A1-Arg-A2-Cys-Tyr-A3-A4-X-A5-A6-Cit-Cys-A7 (I) (wherein A1 represents a hydrogen atom or an arginine, lysine, ornithine, citrulline, alanine residue, etc.; A2 represents an aromatic amino acid residue; A3, A4 and A6 represent an arginine, lysine, ornithine, citrulline or alanine residue, A5 represents a tyrosine, phenylalanine, alanine, naphthylalanine or citrulline residue; A7 represents a lysine or arginine residue in which a carboxyl group may be amidated; X represents a D-ornithyl-proline, prolyl-D-ornithine, D-lysyl-proline residue, etc.; provided that either of A1, A3, A4, A5, A6 and A7 is an alanine residue, etc., or X is citrulline, etc.) or a salt thereof.