A Japanese court on Thursday acquitted the world's longest death row convict, ruling that the evidence in 1966 murder cases was "fabricated." 

In a rare retrial of former professional boxer Iwao Hakamata, 88, a district court in Shizuoka province had been hearing the 1966 quadruple murder case since last October.

The defendant's sister, 91, represented the former boxer at court due to his health condition.

Hakamata has already spent nearly 50 years in jail but was released in 2014 after new evidence had emerged.

The former boxer's case marks the "fifth time in postwar Japan that retrials have resulted in acquittals after the death penalty was given, with the four previous rulings finalized without an appeal by prosecutors," Tokyo-based Kyodo News reported.

A Japanese court had convicted and handed the death sentence to the former boxer in around 1980.

It remains to be seen whether the prosecution, which had sought the death penalty, will challenge the court's ruling.

Hakamata was arrested in 1966 for allegedly killing a senior colleague, along with his wife and their two children, at a miso maker firm in Shizuoka province on Japan's eastern coast.

However, the Tokyo High Court in March last year had noted a "strong possibility" that the main evidence in the case "had been planted by investigators."

Prosecutors had launched the case on the basis of five pieces of clothing that the former boxer wore when the murders happened.

It was with the Japanese Supreme Court's intervention that the retrial began last year.