When C60 was added to synovial fibroblasts, infiltrating lymphocytes, and macrophages, and the inflammatory cytokine production level was measured, the inflammatory cytokine production level was significantly suppressed in all cells. Furthermore, when C60 was added to osteoclast precursor cells and cultured in the presence of osteoclast differentiation-inducing factors, a certain concentration or more of C60 suppressed their differentiation into osteoclasts. Observation of the effect of C60 addition on bone resorption showed that C60 suppressed bone resorption by osteoclasts. In addition, the use of arthritis model animals confirmed in vivo that C60 suppressed inflammatory symptoms, as well as bone resorption and bone destruction by osteoclasts. C60 is effective for treating arthritic diseases such as rheumatoid arthritis through its effects of suppressing osteoclast differentiation, bone resorption, and inflammatory cytokines.