Disclosed is an osteoclastogenesis inhibitory agent which comprises a milk protein fraction having the following properties (1) to (4) and the agent is excellent in an osteoclastogenesis inhibitory effect and 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 80,000 to 85,000 Daltons； (3) the milk protein fraction has a basic amino acid content of 16 to 18 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 0.65 to 0.85； and (4) the milk protein fraction has an osteoclastogenesis inhibitory effect .
