Skin-whitening agent

1. A method for skin-whitening, comprising:

providing a whey protein hydrolyzate having the following characteristics:

(A) the molecular weight is distributed within 10 kDa with a main peak of 200 Da to 500 Da;

(B) the APL (average peptide-chain length) is 2 to 8;

(C) the free amino acid content is 20% or less; and

(D) the antigenicity is one-ten thousandth of antigenicity of β-lactoglobulin or less; and

orally administrating the whey protein hydrolyzate as a skin-whitening agent to a subject desiring skin-whitening, in an amount of at least 5 mg of the whey protein hydrolyzate per day,

wherein the step of providing the whey protein hydrolyzate comprises:

(i) enzymatically decomposing a whey protein as a preliminary decomposition using a protein hydrolyzing enzyme under pH 6 to 10 at 20 to 55° C., and then raising the temperature for further decomposition of the whey protein in step (ii); and

(ii) enzymatically decomposing the hydrolyzed whey protein obtained in step (i) while thermally denaturing the hydrolyzed whey protein using a heat-resistant protein hydrolyzing enzyme under pH 6 to 10 at 50 to 70° C. until a main peak of the hydrolyzed whey protein is reduced to 200 Da to 500 Da as measured when concentrated using a molecular weight cut off in a range of 2 kDa to 10 kDa and in a range of 150 Da to 300 Da, and then heating the obtained product and thereby deactivating the enzyme.

2. The method according to claim 1 , wherein the whey protein hydrolyzate is administrated to the subject at a concentration of 0.01 to 1%.