A novel method of myostatin inhibition is provided.
An antisense oligonucleotide of 15-30 bases or a salt or a solvate thereof, wherein the antisense oligonucleotide has a nucleotide sequence complementary to a target site in exon 1 of the myostatin gene and is capable of inhibiting the production of the mRNA of the myostatin gene. A pharmaceutical drug, a food, a feed, an agent for promoting myocyte proliferation and/or hypertrophy, an agent for increasing muscle mass and/or inhibiting muscle weakness, an agent for inhibiting production of the mRNA of the myostatin gene, and an inhibitor of the function of myostatin, each of which comprises the above antisense oligonucleotide or a salt or a solvate thereof.