The Tokyo High Court backed a lower court's October 2003 conviction of Tomomasa Nakagawa for helping to make the deadly sarin nerve gas used in the subway attack that killed 12 people, and in an earlier attack that killed seven people, said a court spokeswoman who spoke on condition of anonymity citing court protocol.
Nakagawa was also found guilty of participating in other cult murders.
The spokeswoman said she had no other details from the ruling.
More than a dozen death sentences have been handed out to members of the Aum Shinrikyo cult, but none of them have been executed. Former Aum guru Shoko Asahara is on death row for 27 killings, including 12 in the subway attack.
Before the subway attack, the cult amassed an arsenal of chemical, biological and conventional weapons in anticipation of an apocalyptic showdown with the government.
In 2000, Aum renamed itself Aleph but remains under close police surveillance.
Asahara's former top lieutenant, Fumihiro Joyu, left the group and established a 160-member sect of his own, called "Ring of Light," in May. He has denied the group follows Asahara's teachings, but critics say the move is a coverup.