The present invention addresses the problem of providing a bone-strengthening agent that is very safe and useful in preventing or treating a variety of bone diseases such as osteoporosis, fracture, rheumatism, and arthritis. The present invention also provides a bone-strengthening food product, bone-strengthening nutritional composition, bone-strengthening feed, or bone-strengthening pharmaceutical product in which the bone-strengthening agent has been blended. Simultaneous intake of a milk-derived basic protein fraction and/or milk-derived basic protein fraction degradation product and an isoflavone and/or metabolite thereof exhibits a greater synergistic bone-strengthening action than separate intake.
