Judge says man believed to be world's longest-serving death row inmate may have been convicted on falsified evidence

A Japanese man who has spent more than 45 years on death row has been freed after a court ordered a retrial in his murder case, amid suggestions that police investigators fabricated evidence against him.

Iwao Hakamada was sentenced to hang in 1968 for the murders two years earlier of a company president, his wife and their two children in Shizuoka prefecture, central Japan. Hakamada, who was also convicted of burglary and arson in the same case, is thought to be the world's longest-serving death row inmate.

The 78-year-old looked frail and unsteady as he emerged from the Tokyo prison with his sister Hideko, 81, who has campaigned for his release.

Hakamada initially admitted carrying out the murders, but retracted the confession and insisted he was innocent throughout his two-year trial. The supreme court denied his first appeal for a retrial in 1980.

In their most recent appeal to the Shizuoka district court, Hakamada's lawyers said the results of DNA test on items of bloodstained clothing belonging to their client proved the blood was not his.

The presiding judge, Hiroaki Murayama, revoked the death sentence and ordered Hakamada's release pending the retrial. "It is unjust to detain the defendant further, as the possibility of his innocence has become clear to a respectable degree," Murayama said.

Hideko Hakamada with a picture of her brother as a young man. Photograph: Kazuhiro Nogi/AFP/Getty Images

Murayama said he was concerned that investigators could have falsified the evidence against Hakamada. "There is a possibility that [key pieces of] evidence have been fabricated by investigative bodies," Murayama said in his ruling, according to Jiji Press.

Courts in Japan rarely order retrials in cases involving capital punishment. Hakamada will become only the sixth death row inmate in the country to secure a retrial. In the five other cases, defendants were acquitted, while another is still pending.

Ms Hakamada thanked dozens of her brother's supporters who had gathered in front of the court. "Everyone, really, really thank you," she said through a loudspeaker. "This happened thanks to all of you who helped us. I am just so happy."

Before going to the Tokyo prison, she said: "I want to see him as soon as I can and tell him: 'You really persevered.'"

Media reports indicated that prosecutors would appeal against the ruling, ignoring pleas from defence lawyers who pointed to their client's failing health after decades on death row. Members of Hakamada's family say he is displaying symptoms of dementia and suffers from mental health problems.

Campaigners against the death penalty have used Hakamada's case to draw attention to Japan's "secret" executions, accusing authorities of driving prisoners insane and subjecting them to "cruel, inhuman and degrading" treatment.

In its latest annual review of the death penalty around the world, Amnesty International said Japan's use of capital punishment "continued to be shrouded in secrecy".

Death row inmates spend many years in solitary confinement, and only learn of the timing of their execution, by hanging, hours before it takes place.

"The Japanese authorities should be ashamed of the barbaric treatment Hakamada has received," said Roseann Rife, east Asia research director at Amnesty International.

"For more than 45 years he has lived under the constant fear of execution, never knowing from one day to the next if he is going to be put to death. This adds psychological torture to an already cruel and inhumane punishment."

Critics have also voiced concern over the harsh tactics interrogators use to obtain confessions, which are then used as the basis for verdicts in the vast majority of cases. Japan has a conviction rate of about 99%.

"It would be most callous and unfair of prosecutors to appeal the court's decision. Time is running out for Hakamada to receive the fair trial he was denied more than four decades ago," Rife added. "If ever there was a case that merits a retrial, this is it. Hakamada was convicted on the basis of a forced confession and there remain unanswered questions over recent DNA evidence."

One of the three judges who convicted Hakamada has publicly stated he believes he is innocent.

A total of 130 people are awaiting execution in Japan, which has resisted growing calls to abolish the death penalty. Executions resumed in March 2012 after a 20-month gap, with eight people hanged last year, the ninth highest number of any country.

Pro-hanging politicians cite opinion polls in which a majority supports capital punishment, although campaigners say the surveys are worded in such a way as to play on the public's fear of crime.

In a 2010 poll, 86% of respondents said the use of the death penalty was "unavoidable" – a sentiment that strengthened after a doomsday cult carried out a gas attack on the Tokyo subway in 1995, killing 13 people and injuring thousands more.