In Japan, the prison population has been aging at a pace far exceeding the general population. This rapid aging has increased the costs in all aspects of prison administration, which has become a great burden on the Japanese prison system. However, little has been clear about the causes of the current explosion of elderly prisoners. This study explores factors affecting the aging prison population, and examines the extent to which three main possible causes affect the aging prison population: the elderly population growth in general society, increased criminal behavior of elderly people, and legal harshness by the criminal justice organizations, such as kinds of sentences and lengths of imprisonment. This study adopts a longitudinal design, using three kinds of cross-sectional annual data sources aggregated for the whole country of Japan during 20 years from 1988 to 2007. The findings indicate that while the the non-elderly prison population growth has been caused by harsher legal attitudes, the main reason for the elderly prison population growth is increased criminal behavior itself.