RANKL (receptor activator of nuclear factor-κB ligand) critically regulates the differentiation, activation, and survival of the osteoclast, and the hyperactivation of the RANKL-RANK signaling pathway leads to osteoporosis. Denosumab is a complete human monoclonal antibody against human RANKL, and suppresses bone resorption by inhibiting the interaction between RANKL and RANK. Accumulating evidence has demonstrated the clinical usefulness of denosumab on postmenopausal osteoporosis. Although the effectiveness of denosumab on disease activity of rheumatoid arthritis has not been clarified yet, previous studies reported that denosumab significantly increased the bone density of lumbar spine and proximal femur of rheumatoid arthritis patients.