Disclosed is an agent for promoting the differentiation of an osteoblast and inhibiting the differentiation of an osteoclast, which comprises a milk protein fraction having the following chracteristic properties (1) to (4): (1) the milk protein fraction is derived from milk; (2) the milk protein fraction is a fraction that contains a protein component having a molecular weight of 75,000 to 80,000 Daltons; (3) in the constituent amino acid composition of the milk protein fraction, the basic amino acid content is 13 to 15 wt%, and the ratio of the amount of a basic amino acid(s) to the amount of an acidic amino acid(s) [basic amino acid(s)/acidic amino acid(s)] is 0.5 to 0.7; and (4) the milk protein fraction has an activity of promoting the differentiation of an osteoblast and inhibiting the differentiation of an osteoclast.