The present invention relates to a peptide derivative selected from the group consisting ofPEG20k(AL)-β-Ala-Tyr-Nal(1)-Leu-Phe-Arg-Pro-Arg-Asn-NH2,PEG20k(AL)-β-Ala-Tyr-Nal(2)-Leu-Phe-Arg-Pro-Arg-Asn-NH2,PEG20k(AL)-NpipAc-Tyr-Nal(2)-Leu-Phe-Arg-Pro-Arg-Asn-NH2,PEG20k(AL)-NpipAc-Tyr-Nal(2)-Leu-Phe-Arg-Ala-Arg-Asn-NH2,PEG20k(AL)-PEG(2)-Tyr-Nal(2)-Leu-Phe-Arg-NMeAla-Arg-Asn-NH2,PEG20k(AL)-Pic(4)-Tyr-Nal(2)-Leu-Phe-Arg-NMeAla-Arg-Asn-NH2,PEG20k(AL)-Acp-Tyr-Nal(2)-Leu-Phe-Arg-NMeAla-Arg-Asn-NH2, andPEG20k(AL)-β-Ala-Tyr-Nal(2)-Leu-Pya(4)-Arg-Pro-Arg-Asn-NH2or a salt thereof.