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.