A man believed to be the world's longest-serving death row inmate walked free from jail after decades in solitary confinement, in a rare about-face for Japan's rigid justice system. 

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 1966 murder of his boss and the man's family. 

Delivering his ruling, presiding judge Hiroaki Murayama cited possible planting of evidence by investigators to win a conviction as they sought to bring closure to a crime that shocked the country.

"There is possibility that [key pieces of] evidence have been fabricated by investigative bodies," Murayama said in his decision, according to Jiji Press.

The judge also ordered Hakamada's release, saying continued confinement "goes against justice".

Hakamada initially denied accusations that he robbed and killed his boss, the man's wife and 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 emerged a year after the crime and Hakamada's arrest, as key evidence to convict him.

The clothes did not fit him, his supporters said. Later DNA tests found no link between Hakamada, the clothes and the blood stains, lawyers argued.

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.

The decision to retry Hakamada 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. Nearly 130 others are also believed to be on death row.

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 organisation's East Asia research director.

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 a standard of guilt.

Source: Agencies