The purpose of the present invention is to provide a novel muscle atrophy inhibitor containing an ingredient that can be taken safely over an extended period of time. It was discovered that a glycoside of quercetin, which is a type of polyphenol contained in plants and the like, has an inhibitory effect on the expression of myostatin which is involved in muscle atrophy. The present invention provides a novel muscle atrophy inhibitor containing quercetin glycoside, the muscle atrophy inhibitor having exceptional bioavailability and high safety.