Diabetes mellitus, especially type 2 diabetes, has been a major concern among health issues for Japanese policy requiring serious address. In this regard, the increasing number of people with diabetes in Japan over the past few decades has been regarded as a fundamental and evident problem. We herein clarify that the crude diabetes prevalence is likely to have increased mainly as a result of changes in the population distribution, namely, aging of the general population in Japan. We also predict the future trend for the number of people with diabetes according to available concrete evidence in this review.