Iwao Hakamada, 88, was kept on death row for more than 50 of those years before being acquitted in a retrial last month.

Shizuoka District Court said police and prosecutors had collaborated to fabricate and plant evidence against the former boxer, and used violence to force him to confess.

Shizuoka Prefectural Police Chief Takayoshi Tsuda visited Mr Hakamada at his home to apologise in person on Monday.

Standing in front of Mr Hakamada, the officer bowed deeply and said: "We are sorry to have caused you unspeakable mental distress and burden for as long as 58 years.

"We are terribly sorry."

Mr Hakamada, who struggles to hold conversations due to his mental condition from decades on death row, said: "What it means to have the authority... Once you have the power, you're not supposed to grumble."

Mr Hakamada's 91-year-old sister, Hideko Hakamada, who had stood by her brother through the long process to clear his name and now lives with him, thanked the police chief for visiting them.

She told reporters there was no point complaining to him after all these years, as Chief Tsuda "was not involved in the case and he only came here as his duty".

She said she agreed to the visit "because I wanted [my brother] to have a clear break from his past as a death row inmate."

Hakamada was sentenced to death in 1968 for killing his former boss, an executive at a miso bean paste company, his wife, two of their children and setting fire to their home two years before in Hamamatsu, central Japan.

He avoided execution because of the country's lengthy appeal and retrial process, which meant he'd been in jail for 27 years by the time his first appeal for a retrial was turned down.

His second appeal for a retrial, filed by his sister in 2008, was granted in 2014.

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Mr Hakamada was the world's longest-serving death row prisoner and only the fifth death row inmate to be acquitted in a retrial in post-war Japan, where criminal trials take years and retrials are extremely rare.

Questions arose over blood-stained clothes investigators said belonged to him, which were found more than a year after his arrest, hidden in a tank of fermented soybean paste, or miso.

Blood samples did not match Mr Hakamada's DNA, and the trousers that prosecutors submitted as evidence were too small for him.