An 88-year-old man who was convicted of murder in 1968 and has been on death row for 46 years was acquitted on Thursday.
A Japanese court reportedly ruled that evidence that was used to convict him had been fabricated.
Former boxer Iwao Hakamada was believed to be the world's longest-serving death row prisoner.
His health was too weak to be in court to learn about the outcome of his retrial.
"Everyone -- we won the acquittal, it's all thanks to your support," his 91-year-old sister Hideko she said outside the Shizuoka District Court, the AFP reported.
Hakamada had been convicted of robbing and killing his boss, the man's wife and their two teenage children.
The court ruled that investigators had tampered with evidence and used "inhumane interrogations" to force him to confess he carried out the killing.
Prosecutors have two weeks to appeal the verdict, according to Japanese media.
Japan and the United States are the only major industrialized democracies that still have capital punishment.
Hakamada's lawyer said his client sometimes seems like he "lives in a world of fantasy" after decades of detention, mostly in solitary confinement.
A retrial was granted in 2014 and Hakamada was released from prison, although legal wrangling meant the proceedings only began last year.
Amnesty International said Hakamada had endured "almost half a century of wrongful imprisonment and a further 10 years waiting for his retrial."
