The sister of an 87-year-old man convicted of killing a family of four nearly six decades ago has thanked supporters one day after a court in Tokyo ordered a retrial in his case.

The Tokyo High Court on Monday granted a retrial for Hakamada Iwao, saying there is reasonable doubt about key evidence based on which he was found guilty of the 1966 murders in central Japan.

On Tuesday, his elder sister Hakamada Hideko and lawyers met supporters in Tokyo.

She said her hardship is nothing compared to that of her brother, who was in prison for 48 years. She said, occasionally in tears, that she is pleased that her 57 years of fighting have been rewarded.

Lawyer Hazama Mitsuhiro said the high court decision has paved the way for Hakamada's acquittal. He also said prosecutors must not challenge the decision.

Hakamada Iwao was at his home in Hamamatsu City, central Japan, when the court made the decision. He reportedly had a Japanese-style celebratory meal of red bean rice and sea bream on Tuesday morning.

His supporters say he looked a bit surprised while reading the front page of a newspaper carrying an article on him. In the afternoon, he went out in a car driven by a supporter for his daily routine.

Later in the day, after returning from Tokyo, his sister told him that he won the court battle, showing him a copy of the court's decision.