An 86-year-old death row inmate who had been appealing for a retrial for decades died early Wednesday at the Tokyo Detention House, officials said.
Detention house officials said he apparently died of chronic kidney failure.
They refused to release the man's name. People close to the deceased said he is Tsuneki Tomiyama, who was convicted 27 years ago of murdering a friend.
According to the officials and people close to the deceased, Tomiyama was suffering from chronic kidney disease and high blood pressure, and had been undergoing dialysis in a medical ward at the detention house since October.
A patrolling guard noticed around 1 a.m. Wednesday that Tomiyama would not respond to his call and called a doctor, the officials said. He was confirmed dead at 1:48 a.m.
An examination by a coroner indicated nothing suspicious about the death, the detention house said.
Tomiyama was convicted of killing a male friend in Ibaraki Prefecture in 1963 by giving him a capsule of cyanide compound.
Tomiyama had taken out a life insurance policy on his friend and had hoped to cash in on the benefits.