A 68-year-old death row inmate convicted of serial bombings at Japanese companies in 1974 and 1975 died Wednesday at the Tokyo detention house, the Justice Ministry said.

Acting as a member of an extremist group, Masashi Daidoji was convicted of bombings that included the August 1974 attack on the headquarters building of Mitsubishi Heavy Industries Ltd. in Tokyo that killed eight people. Daidoji died of multiple myeloma, a blood cancer, the ministry said.

Daidoji was among leftist militants arrested in 1975 on a charge of conducting a bombing campaign against Japanese companies to condemn Japan's military and commercial advance in East Asia before and after World War II.

Although the Supreme Court sentenced him to death in 1987, he had appealed multiple times for a retrial, arguing that a new examination of the bombs used in the Mitsubishi Heavy attack had shown he had no intention of killing.

The extremist group also targeted the headquarters of trading house Mitsui & Co., construction firm Taisei Corp. and synthetic fibers maker Teijin Ltd.

In connection with the serial bombings, nine persons were arrested and one committed suicide. Six, including Daidoji, were convicted.

Some of the arrested were released as extra-judicial measures taken after the Japanese Red Army hostage crisis in Kuala Lumpur, Malaysia, in 1975 and the hijacking of a Japan Airlines jetliner in the Bangladesh capital Dhaka in 1977.

Daidoji along with Toshiaki Masunaga, also sentenced to death for the 1974 bombing, had sought compensation from the state, claiming that the Tokyo detention house did not allow the supporters to provide the two with letters, cash and other materials in 2004.

Such a practice was later corrected and a district court ordered the state in 2008 to pay the two a symbolic amount of money of 10,000 yen each.