Japan has been assumed to be a country with a low prevalence of rheumatoid arthritis (RA) . However, when comparing the RA prevalence among different countries in the middle age or in the older ages, in which RA predominantly occurs, RA prevalence of Japan is somewhat lower than that of United Kingdom, Finland, and United States, but is similar to that of Norway, France, and Greece. Several reports showed that RA incidence has been decreasing recently. This indicates that non-genetic factors are related to the occurrence of RA and that such factors have been decreasing with time.