Disclosed is a bone-reinforcing agent which comprises a milk protein fractionhaving the following properties (1) to (4): (1) the milk protein fraction isderived frommilk； (2) the milk protein fraction is a fraction containing a proteincomponent having amolecular weight of 6,000 to 150,000 Daltons； (3) the milk protein fractionhas a basicamino acid content of 12 to 14 wt% in the constitutional amino acidcomposition, and theratio of the amount of a basic amino acid(s) to the amount of an acidic aminoacid(s) is0.5 to 0.7； and (4) the milk protein fraction has an activity of acceleratingthecalcification in an osteoblast.
