Iwao Hakamada, 88, had spent 48 years behind bars, more than 45 of them on death row, longer than any other inmate.

The ex-boxer was sentenced to death in 1968 for killing his former boss, his wife, and two of their children and setting fire to their home.

He was acquitted on Thursday by a court in Shizuoka in central Japan, after the presiding judge, Koshi Kunii, said he wasn't guilty and evidence used against him had been made up, Japanese public broadcaster NHK said.

Hakamada originally denied being behind the murders, before confessing, which he later said he was forced to do after a violent interrogation by police.

Questions arose over blood-stained clothes investigators said belonged to him, which were found more than a year after his arrest, hidden in a tank of fermented soybean paste, or miso.

In 2023, a Tokyo High Court accepted evidence that clothing soaked in miso for more than a year turns too dark for bloodstains to be seen and admitted the evidence may have been concocted by investigators.

Furthermore, blood samples did not match Hakamada's DNA, and the trousers that prosecutors submitted as evidence were too small for him.

His planned execution was delayed by lengthy appeals and the retrial process, which meant he'd been in jail for 27 years by the time his first appeal for a retrial was turned down.

Last year the court changed its verdict, ruling in favour of his second appeal, organised by his 91-year-old sister, Hideko Hakamada, in 2008.

That ruling led to the latest retrial, which began in October.

Hakamada hasn't been in prison for 10 years as he was released in 2014 when a court ordered a retrial after new evidence suggested investigators fabricated evidence used against him, but he was not acquitted then.

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After his release, Hakamada served his sentence at home because his frail health and age made him a low risk for escape.

At a final hearing at the Shizuoka court in May before Thursday's decision, prosecutors again demanded the death penalty, triggering criticism from rights groups that prosecutors were trying to prolong the trial.

He is the fifth death-row convict to be found not guilty in a retrial in Japan since 1945.