The human rights group Amnesty International called Wednesday on Japan to launch a "thorough, public and independent" investigation into alleged abuses of prisoners at Nagoya Prison.
"All those responsible for such abuses should be brought to justice," Amnesty International said in a statement.
The London-based rights group cited two publicly disclosed cases of abuse at Nagoya Prison: a 30-year-old man who received medical treatment in September after he was manacled and put in solitary confinement, and the death of a 49-year-old inmate in May while in solitary confinement.
Amnesty said it also wants the government to investigate prisoner complaints regarding the use of force and ill-treatment by prison officials.
"Japan should ensure that the rights of all prisoners and detainees -- as guaranteed in international human rights standards to which Japan is a state party -- are protected," Amnesty said.
The organization urged Japanese authorities to establish an independent body to inspect prisons, monitor the treatment of inmates and the general conditions of detention.
The inspection body should be able to speak privately to prisoners, and report publicly on its findings. Doctors and psychiatrists should be members of this body, Amnesty said.
Amnesty underscored the lack of transparency regarding the internal regulations at penal institutions in Japan, saying prison officials are given wide discretionary powers to decide secretly enforced rules at their facilities.
All detention facilities operate under extremely strict disciplinary regimes with inmates forced to comply with arbitrary rules rigorously enforced by guards, the group said.
Amnesty called the treatment of inmates in "protection cells" -- where the group says prisoners are held in restraining devices, consisting of manacles and leather harnesses, that are kept on even while they eat -- "cruel, inhuman and degrading," and said such treatment must be stopped.