Provided is a muscle-building agent comprising a whey protein hydrolyzate having a muscle increasing effect which is highly safe, an aqueous solution of which is highly transparent and which can be used in various products without limitation because of having little bitterness and thus not affecting the taste of the products.A muscle-building agent comprising a whey protein hydrolyzate which has a molecular weight distribution of 10 kDa or less, shows a main peak at 200 Da to 3 kDa, has an average peptide chain length (APL) of 2 to 8, contains free amino acids in an amount of 20% or less relative to the total components thereof and shows an antigenicity that is 1/100,000 times or less than the antigenicity of β-lactoglobulin.
