Peptide or pharmaceutically acceptable salt thereof, or prodrug thereof

A peptide or a pharmaceutically acceptable salt of the peptide, or a prodrug thereof, the peptide

• satisfying the following (a) or (b) and having a number of amino acid residues of 20 or less:
• (a) the peptide comprising any one of the amino acid sequences represented by SEQ ID NO:
• 2 to 9, SEQ ID NO:
• 11 to 20, SEQ ID NO:
• 22 to 38, SEQ ID NO:
• 380 to 385, SEQ ID NO:
• 573 to 580, and SEQ ID NO:
• 583 to 595;
• or
• (b) the peptide comprising one amino acid sequence in which one amino acid residue of X2 to X4 and X6 to X16 in the amino acid sequences of the above (a) is substituted or deleted and having a myostatin inhibitory activity
• wherein, SEQ ID NO:
• 585 to 587 and SEQ ID NO:
• 593 to 595 may represent a cyclic peptide in which a disulfide bond is formed between side-chain SH groups of two cysteine residues.