A milk protein fraction having following properties (1) to (4) is excellent inbone resorption inhibitory effect, and is useful for preventing or treatingbone diseases:(1) the milk protein fraction is derived from milk；(2) the milk protein fraction is a fraction containing a protein having amolecular weight of 12,000 to 16,000 Daltons；(3) the milk protein fraction has a basic amino acid content of 18 to 20 wt%inthe constitutional amino acid composition, and the ratio of the amount of abasic aminoacid(s) to the amount of an acidic amino acid(s) is 0.7 to 0.9； and(4) the milk protein fraction has a bone resorption inhibitory effect.
