As the aging society with low birth rate progresses, the burden of care for the dementia elderly increases. Thus, an increasing attention has been paid to the epidemiology of dementia in Japan. This phenomenon is also observed in many developing countries all over the world. In this paper, the author reports the prevalence of dementia among the elderly people aged 65 years and older in Japan using the data from a recent nation-wide survey. According to the results of this survey which was conducted at seven sites in Japan, the prevalence rate was estimated to be 15.75% (95% CI: 12.4-22.2%) which was much higher than that had been estimated before. Alzheimer disease is the most common illness that causes dementia, and followed by vascular dementia and Lewy body dementia. As the limitation of this nation-wide survey, no study was conducted in urban area with low percentage of elderly in a population. Thus, additional studies are ongoing into the prevalence of dementia in urban areas.