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.