A former professional boxer and death-row inmate accused of a 1966 quadruple murder in central Japan is innocent, his elderly sister said Friday, in the first hearing of his retrial that is likely to lead to his exoneration.

Iwao Hakamata, 87, was exempted from attending the retrial at the Shizuoka District Court, as his mental state has deteriorated after spending nearly half a century behind bars before new evidence led to his release in 2014.

"I will plead my brother's innocence for him. Please grant Iwao true freedom," 90-year-old Hideko said, appearing at the trial in his place.