Japan's highest court has overturned a ruling blocking the retrial of a man described as the world's longest-serving death row inmate, raising new hope for the 84-year-old, his lawyer said Wednesday.

Iwao Hakamada has lived under a death sentence for more than half a century after being convicted of robbing and murdering his boss, the man's wife, and their two teenaged children.

But he and his supporters say he confessed to the crime only after an allegedly brutal police interrogation that included beatings, and that evidence in the case was planted.

He tried to retract his confession, but was sentenced to death in 1968, with the verdict confirmed by the Supreme Court in 1980.

However, in a rare about-face for Japan's rigid justice system, a district court in the central city of Shizuoka in 2014 granted his request for a retrial.

The court said investigators could have planted evidence and ordered the former boxer freed, adding it was "unbearably unjust" to keep him detained pending the new trial.

Prosecutors appealed that ruling, and won at the Tokyo High Court, prompting Hakamada to move the case to the Supreme Court, which on Wednesday reversed the high court decision.

"The Supreme Court made a decision today to uphold a retrial by overturning the decision by the Tokyo High Court to dismiss the request for retrial," Hakamada's lawyer Yoshiyuki Todate wrote on his blog.

"The fact that a path for the resumption of a retrial was not cut off is very welcome. My hands are still shaking after hearing this. I'm really, really glad."

Iwao Hakamada was convicted of murdering four members of the same family, but claims he confessed after a brutal police interrogation
Iwao Hakamada was convicted of murdering four members of the same family, but claims he confessed after a brutal police interrogation AFP / Kazuhiro NOGI

Local media said the issue would now return to the Tokyo High Court for fresh deliberation, with a retrial still not guaranteed.

Hakamada's sister Hideko nonetheless welcomed the news.

"I'm happy. I didn't expect we'd have the top court decision this year," she told national broadcaster NHK.

"It's a Christmas gift."

Another of Hakamada's lawyers, Hideyo Ogawa, told reporters he felt the Supreme Court had "correctly understood our claim."

"I have high hopes that we'll be able to bring (this case) for retrial."

Supporters say nearly 50 years of detention, mostly in solitary confinement with the ever-present threat of execution looming over him, took a heavy toll on Hakamada's mental health.

In an interview with AFP in 2018, the former boxer said he felt he was "fighting a bout every day".

Japan is the only major industrialised democracy other than the United States to carry out capital punishment, which still enjoys broad public support, although debate on the issue is rare.