Disclosed is a bone-strengthening agent which comprises a milk protein fractionhaving the following properties (1) to (4): (1) the milk protein fraction is derivedfrom milk； (2) the milk protein fraction is a fraction containing a protein componenthaving a molecular weight of 6,000 to 150,000 Daltons； (3) in the constitutionalamino acid composition of the milk protein fraction, the basic amino acid contentis 12 to 14 wt%, and the ratio of the amount of a basic amino acid(s) to the amountof an acidic amino acid(s) is 0.5 to 0.7； and (4) the milk protein fraction has anactivity of accelerating the calcification in an osteoblast.
