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