Using metabolic balance test, bone mineral density measurement, animal models of osteoporosis, bone cell culture, we investigated the relationship between osteoporosis and the metabolism of calcium and bone. We found that the calcium intake was rather low in 20 healthy adults (480 mg per day); the average intestinal net absorption rate(%) of calcium in 124 healthy adult volunteers was 28.9(%), which increased gradually in recent 20 years (from 17.1% in 1978 to 40.6% in 1977), owing to improvement of dietetic structure. We conclude that the insufficiency of sex hormone and calcium are the main causes of osteoporosis. In bone cell culture, the whole process of bone remodelling was observed for 35 days continuously. Calcitonin damaging osteoclast was also investigated. Thus, we suggest new concept of bone remodelling coupling.