Disclosed is an osteoblast differentiation promoting and osteoclast differentiation inhibiting agent which comprises a milk protein fraction having the following characteristics (1) to (4): (1) the milk protein fraction is derived from milk； (2) the milk protein fraction is a fraction containing a protein component having a molecular weight of 75,000 to 85,000 Daltons； (3) the milk protein fraction has a basic amino acid content of 13 to 15 wt% in the constitutional amino acid composition, and has a basic amino acid/acidic amino acid of 0.5 to 0.7； and (4) the milk protein fraction has an osteoblast differentiation promoting effect and an osteoclast differentiation inhibiting.
