The purpose of the present invention is to provide: a bone-strengthening agent which is useful for the prevention or treatment of various bone diseases such as osteoporosis, bone fracture, rheumatism and arthritis； a bone-strengthening product such as a food, a beverage, a feed and a medicine, which contains the bone-strengthening agent. A bone-strengthening agent, which is characterized by comprising a milk-derived basic protein fraction and/or a decomposition product of a milk-derived basic protein fraction and a hydrolysate of a whey protein as active ingredients, is useful for the prevention or treatment of various bone diseases such as osteoporosis, bone fracture, rheumatism and arthritis.
