A milk protein fraction having following properties (1) to (4) is excellent in bone resorption inhibitory effect, and is useful for preventing or treating bone diseases: (1) the milk protein fraction is derived from milk； (2) the milk protein fraction is a fraction containing a protein having a molecular weight of 12,000 to 16,000 Daltons； (3) the milk protein fraction has a basic amino acid content of 18 to 20 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 0.7 to 0.9； and (4) the milk protein fraction has a bone resorption inhibitory effect.
