Bone homeostasis is maintained by delicate balance between bone resorption by osteoclasts and bone formation by osteoblasts. Bisphosphonates are widely used nowadays by suppressing bone resorption to treat patients with osteoporosis, which results from high bone turnover, causing excessive bone resorption phase. While bisphosphonates increase bone mineral density and improve back pain due to spinal compression fracture, they may have some problems such as osteonecrosis of jaw and excessive suppression of bone turnover. Cathepsin K inhibitor, which has a new mechanism in addition to function of suppressing bone resorption, is recently focused. Cathepsin K is a protease which specifically expresses in osteoclasts and plays an important role in resolution of bone collagen. Cathepsin K inhibitor has a potential of inhibiting bone resorption without suppressing bone formation and could be an attractive therapeutic target of osteoporosis.