Disclosed is a bone-reinforcing agent which comprises a milk protein fraction having the following properties (1) to (4): (1) the milk protein fraction is derived from milk； (2) the milk protein fraction is a fraction containing a protein component having a molecular weight of 6,000 to 150,000 Daltons； (3) the milk protein fraction has a basic amino acid content of 12 to 14 wt % in the constitutional amino acid composition, and the ratio of the amount of a basic amino acid(s) to the amount of an acidic amino acid(s) is 0.5 to 0.7； and (4) the milk protein fraction has an activity of accelerating the calcification in an osteoblast.
