PURPOSE:To prepare a bone-fortifying food, pharmaceutical or feed enabling the absorption of large amounts of whey protein and peptide and effective for fortifying the bone by using a water-soluble fraction of whey protein as a component. CONSTITUTION:Whey is a transparent yellowish green liquid obtained as a by-product of a cheese or casein preparation process and produced by adding an acid or rennet to milk or defatted milk and removing the coagulum from the mixture. Whey protein is a water-soluble protein produced by the ultrafiltration, reverse osmosis, chromatography, dialysis, etc., of the whey. The protein to be used in the present product is preferably a water-soluble fraction of a whey protein having a molecular weight of 10,000-45,000 and an isoelectric point of PI 3-8. In the case of performing an enzymatic decomposition with a proteinase, the solution or suspension of the protein is added with a proteinase and subjected to enzymatic decomposition. A proteinase- decomposition product having a molecular weight of about 20,000-300 can be produced by adjusting the solution or suspension of the protein to the optimum pH and optimum temperature for the enzyme, adding the enzyme to the solution, etc., and subjecting the solution to enzymatic reaction for a prescribed time in a stationary state or under stirring.