A method of inhibiting the function of myostatin 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 sequence in exon 3 of the myostatin gene and is capable of allowing the expression of a splicing variant of myostatin. A pharmaceutical drug, a food, a feed, an agent for promoting myocyte proliferation and/or hypertrophy, an agent for increasing muscle mass and/or suppressing muscle weakness, an agent for switching the splicing of the myostatin gene from production of myostatin to production of a splicing variant thereof, an agent for decreasing myostatin signaling, and an anticancer agent, each of which comprises the above antisense oligonucleotide or a salt or a solvate thereof.