A muscle-building agent comprising a whey protein hydrolyzate having a molecular weight distribution of 10 kDa or less, a main peak of 200 Da to 3 kDa, an average peptide length (APL) of 2 to 8, a free amino acid content of 20% or less, and antigenicity equal to or less than 1/100,000th of that of ²-lactoglobulin is provided. The whey protein hydrolyzate is very safe, exhibits high transparency when dissolved in water, has low bitterness, can be used for an arbitrary product, and exhibits a muscle-building effect.
