A Japanese court has ordered the release of the world's longest-serving death row inmate, saying investigators had probably made up evidence in a murder case that left him behind bars for nearly half a century.

Shizuoka District Court suspended the death sentence for 78-year-old Iwao Hakamada, a former professional boxer convicted of the 1966 murder of a family, and ordered a retrial.

More than 45 of the 48 years he has spent in jail have been on death row, making Hakamada the longest-serving such inmate, according to Guinness World Records.

Hakamada was sentenced to death in 1968, but was not executed because of a lengthy appeals process. It took 27 years for the supreme court to deny his first appeal for a retrial. He filed a second appeal in 2008, and the court finally ruled in his favour.

The court said DNA analysis obtained by Hakamada's lawyers suggested that investigators had fabricated evidence. It ordered a retrial in the case, making Hakamada only the six death row inmate to get a retrial in Japan's history of postwar criminal justice. Four of the previous inmates were acquitted in their retrials, while the fifth case is still pending.

Thursday's ruling underscores Japan's much-criticized closed interrogations, which rely heavily on self-confession. Hakamada had confessed in a closed interrogation.

Hakamada was convicted of killing a company manager and his family and setting fire to their central Japan home, where he was a live-in employee.