The world's longest-serving death row prisoner hears from a Japanese court on Thursday if he will again face execution or finally be acquitted, a decade after obtaining a retrial of his murder conviction.

Iwao Hakamada, 88, was jailed under the death penalty for 46 years until he was freed in 2014 pending a retrial.

The former boxer was first convicted in 1968 of killing his boss, the man's wife and their two teenage children.

But over the years, questions arose over fabricated evidence and coerced confessions, sparking scrutiny of Japan's justice system, which critics say holds suspects "hostage".

"For so long, we have fought a battle that has felt endless," Hakamada's sister Hideko, 91, told reporters in July. "But this time, I believe it will be settled".

Prosecutors meanwhile have said they remain convinced of his guilt "beyond reasonable doubt".

Japan is the only major industrialised democracy other than the United States to retain capital punishment, a policy that has broad public support.

Hakamada is the fifth death row inmate granted a retrial in Japan's post-war history. All four previous cases resulted in exoneration.

After decades of detention, mostly in solitary confinement, Hakamada's health has deteriorated and he sometimes seems like he "lives in a world of fantasy", according to his lead lawyer Hideyo Ogawa.

Speaking to AFP in 2018, Hakamada underlined his ongoing battle to obtain an acquittal, saying he felt he was "fighting a bout every day".

"Once you think you can't win, there is no path to victory," he said.

Blood and miso

Although the Supreme Court upheld Hakamada's death sentence in 1980, his supporters fought for decades to have the case reopened.

A turning point came in 2014 when a retrial was granted on the grounds that prosecutors could have planted evidence, and Hakamada was released from prison.

Legal back-and-forth, including a pushback by prosecutors, meant it took until last year for the retrial to begin.

Hakamada initially denied having robbed and murdered the victims, but confessed following what he later described as a brutal police interrogation that included beatings.

Central to the trial is a set of blood-stained clothes found in a tank of miso -- fermented soybean paste -- a year after the 1966 murders, used as evidence to incriminate Hakamada.

The defence argues investigators likely set up the clothes, as the red stains on them were too bright, but prosecutors say their own experiments show the colour is credible.

Hakamada's supporters and rights groups say his saga exposes Japan's flawed justice process and the cruelty of the death penalty.

In Japan, death row prisoners are notified of their hanging a few hours in advance.

The case is "just one of countless examples of Japan's so-called 'hostage justice' system", Teppei Kasai, Asia programme officer for Human Rights Watch, told AFP.

"Suspects are forced to confess through long and arbitrary periods of detention" and there is often "intimidation during interrogation", he said.

Hakamada's defence team has petitioned the Shizuoka prosecution office to let a not-guilty verdict stand if that is how the court rules on Thursday.

"We told prosecutors that the onus is on them to put an end to this 58-year-old case", Ogawa told reporters this month.