The Shizuoka District Court has acquitted an 88-year-old man who was sentenced to death in 1968 over the murder of a family of four in central Japan two years earlier.

Hakamada Iwao claimed innocence, and the court granted him a retrial in 2014, leading to his release after 48 years.

The process leading up to the retrial's hearings was prolonged as prosecutors appealed the court's decision, but the Tokyo High Court gave the green light last year.

On September 26, Hakamada's elder sister Hideko came to the Shizuoka court hearing on her brother's behalf as the defendant was exempted from court proceedings due to his mental condition.

In the retrial, the biggest point of contention was over five items of clothing that were said to be found more than a year after the incident.

The court said investigators had tampered with the clothes that Hakamada supposedly wore at the time of the crime, including smearing blood on them.

It said the investigators also fabricated a piece of cloth found at Hakamada's home that was said to be the same as the fabric of the bloodstained pants.

The court said an interrogation report in which Hakamada confessed to the crimes was effectively made up as it was a result of questioning conducted in an inhumane manner.

After reading out the ruling, presiding Judge Kunii Koshi apologized to Hideko that the court process took time.

Hakamada spent 46 years on death row, the longest in the world.

The Shizuoka District Public Prosecutors Office said it will respond appropriately after examining the ruling and consult with higher authorities on whether to appeal.

At a press conference, Hideko expressed appreciation and said she was moved to tears when she heard the ruling.