The rooms are filled with elderly residents, their hands wrinkled and backs bent. They shuffle slowly down the corridors, some using walkers. Workers help them bathe, eat, walk and take their medication.

But this isn’t a nursing home – it’s Japan’s largest women’s prison. The population here reflects the aging society outside, and the pervasive problem of loneliness that guards say is so acute for some elderly prisoners that they’d prefer to stay incarcerated.

“There are even people who say they will pay 20,000 or 30,000 yen ($130-190) a month (if they can) live here forever,” said Takayoshi Shiranaga, an officer at Tochigi Women’s Prison located north of Tokyo, during an extremely rare visit granted to CNN in September.