About 1,300 people, or some 14 percent, of 9,700 inmates in Japan aged 60 or older showed signs of dementia as of the end of 2014, according to a Justice Ministry study.
This is the first time the ministry has investigated the number of prisoners that have dementia.
Given that the proportion of elderly inmates has been increasing in Japan and that the number of prisoners with dementia is expected to rise further, the ministry sees a need to assess the situation and is considering countermeasures, it said.
The ministry said Tuesday the number of inmates aged 65 or older surged about 4.6-fold between 1995 and 2014.
There were some reports that elderly inmates were unable to function well in a group, prompting calls for measures to deal with the situation.
Against this background, the ministry carried out simple intelligence tests from January to February last year on 451 prisoners aged 60 or older who were selected at random.
In the tests, 59 inmates, or about 14 percent, showed signs of dementia. Among those aged 65 or older, the proportion was higher at 17 percent.
The proportions were, however, similar to figures for Japanese society as a whole, a ministry official said.
In a bid to slow the progress of dementia among elderly inmates, the ministry plans to provide more health care instructions to them. It will also provide dementia lectures for prison officials.