The Tokyo District Court handed the sentence to Naoko Kikuchi, 42, for her role in a parcel bombing at the Tokyo metropolitan government in 1995 which seriously injured one official.
Kikuchi, who had been one of only two remaining members of the Aum Supreme Truth doomsday cult at large, was arrested in June 2012. Less than two weeks later, the final fugitive was also arrested.
The cult was led by Shoko Asahara, a partially blind guru who preached a blend of Buddhist and Hindu dogma mixed with apocalyptic messages, and developed an obsession with sarin gas, becoming paranoid that his enemies would attack him with it.
Asahara was arrested at a commune near Mount Fuji two months after the attack on Tokyo and sentenced to hang, having been convicted of crimes resulting in multiple deaths. He remains on death row, along with 12 others.