Osteoclasts regulate homeostasis of bone development. A defect in osteoclast development results in osteopetrosis. Recently, the involvement of several molecules in osteoclast development has been found. Osteoclast-associated receptor (OSCAR) is one of such molecules critical for osteoclast differentiation. However, it remains unclear how OSCAR transduces signals for osteoclast differentiation. Here, we found that the FcRgamma chain, a signal transducing adaptor molecule for Fc receptors, is associated with OSCAR and is involved in the cell surface expression of OSCAR. Furthermore, FcRgamma is required for signal transduction by OSCAR. These findings suggest that the FcRgamma-mediated signal transduction by OSCAR is involved in osteoclast differentiation.