Provided is 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 comprises proteins having a molecular weight of 5 75,000 to 80,000 daltons determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), (3) the milk protein fraction comprises 13 to 15 wt% of basic amino acids in the constituent amino acid composition, and has a basic amino acid/acidic amino acid ratio of 0.5 to 0.7, and (4) the milk protein fraction has an osteoblast differentiation promoting effect and an osteoclast differentiation inhibiting effect.
