A slightly unsteady-looking Iwao Hakamada, 78, emerged from the Tokyo prison with his campaigning sister after Shizuoka District Court in central Japan ordered a fresh trial over the grisly 1966 murder of his boss and the man's family.
Presiding judge Hiroaki Murayama said he was concerned that investigators could have planted evidence to win a conviction almost half a century ago as they sought to bring closure to a crime that had shocked the country.
"There is a possibility that (key pieces of) evidence have been fabricated by investigative bodies," Murayama said in his ruling.
Shizuoka prosecutors, who have three days to appeal the decision, told Japanese media that the court's decision was "unexpected".
Apart from the United States, Japan is the only major industrialised democracy to carry out capital punishment, a practice that has led to repeated protests from European governments and human rights groups, who say the justice system is heavily skewed in prosecutors' favour.
Hakamada is the sixth person since the end of World War II to receive a retrial after having a death sentence confirmed, and his case will bolster opponents of capital punishment.
Of the past five former death-row inmates who received retrials in Japan, four were subsequently cleared. Higher courts threw out a retrial motion for the fifth prisoner, although his lawyers have submitted a fresh request for a retrial with new evidence.
After his arrest, Hakamada initially denied accusations that he robbed and killed his boss, the man's wife and their two children before setting their house ablaze.
But the former boxer, who worked for a bean-paste maker, later confessed following what he subsequently claimed was a brutal police interrogation that included beatings.
He retracted his confession, but to no avail, and the supreme court confirmed his death sentence in 1980.- Doubts over evidence -
Prosecutors and courts had used blood-stained clothes, which only emerged a year after the crime and his arrest, as key evidence to convict Hakamada.
The clothes did not fit him, his supporters said. The blood stains appeared too vivid for evidence that was discovered so long after the crime. Later DNA tests found no link between Hakamada, the clothes and the blood stains, his supporters said.
But the now-frail Hakamada remained in solitary confinement on death row, regardless.
His supporters and some lawyers, including the Japan Federation of Bar Associations, have loudly voiced their doubts about the evidence, the police investigations and the judicial logic that led to the conviction.
Even one of the judges who originally sentenced Hakamada to death in 1968 has said he was never convinced of the man's guilt, but could not sway his judicial colleagues who out-voted him.
Japan has a conviction rate of around 99 percent and claims of heavy-handed police interrogations persist under a long-held belief that a confession is the gold standard of guilt.
The decision to grant Hakamada a retrial came as Amnesty International issued its annual review of reported executions worldwide, which showed Japan killed eight inmates in 2013, the ninth-largest national tally in the world.
Hakamada's sister Hideko, 81, who has passionately campaigned for a retrial for decades, thanked dozens of supporters who gathered in front of the court house.
"Everyone, really, really thank you," she said through a loud speaker in front of hordes of journalists and supporters. "This happened thanks to all of you who helped us. I am just so happy."
Hakamada seems to have developed psychological illnesses after decades in solitary confinement, Hideko said in an interview last year.
"What I am worried about most is Iwao's health. If you put someone in jail for 47 years, it's too much to expect them to stay sane," Hideko said in the interview.
Amnesty, which has championed Hakamada's cause and says he is the world's longest-serving death row detainee, called on prosecutors to respect the court's decision.
"It would be most callous and unfair of prosecutors to appeal the court's decision," said Roseann Rife, the organization's East Asia research director.
"Time is running out for Hakamada to receive the fair trial he was denied more than four decades ago," she said.