Nearly nine years ago, new mechanism that bone secretes a hormone called osteocalcin and regulates glucose/energy metabolism was discovered. To date the study of osteocalcin as the bone hormone was progressed well, and the new roles in various tissues, such as glucose metabolism, male fertility, and development of the brain, are demonstrated. On the other hand, signaling pathway of osteocalcin has not yet been fully understood, though its receptor was reported. This review focuses on the diverse roles of osteocalcin and also on the future task that should be solved.