Disclosed is a milk protein fraction having the characteristic properties (1)to (4) shown below, which has an excellent inhibitory activity on bone resorptionand is therefore useful for the prevention or treatment of a bone disease: (1)the milk protein fraction is derived from milk； (2) the milk protein fractionis a fraction that contains a protein having a molecular weight of 12,000 to 16,000Daltons； (3) in the constituent amino acid composition of the milk protein fraction,the basic amino acid content is 18 to 20 wt%, and the ratio of the amount of a basicamino acid(s) to the amount of an acidic amino acid(s) [basic amino acid(s)/acidicamino acid(s)] is 0.7 to 0.9； and (4) the milk protein fraction has an activityof inhibiting bone resorption.
