Gen Sekine, a former pet breeder on death row for killing four people in Saitama Prefecture in 1993, died Monday while in detention, a person familiar with his condition said.
The 75-year-old inmate — who was convicted of conspiring with his former wife Hiroko Kazama to kill three people over a financial dispute stemming from his dog breeding business — is believed to have died of an illness, the source said.
Kazama, 60, is also on death row.
Sekine, who was also convicted of a separate killing the same year, died at the Tokyo Detention House on Monday morning. He had collapsed there in November, according to the source.
In 1993, he murdered a 39-year-old company employee, a senior member of a crime syndicate and the man's driver by making them swallow poison capsules. He then dismembered their bodies before incinerating and abandoning the remains, according to a court ruling.
In the separate case, Sekine murdered a 54-year-old woman after selling dogs of foreign origin to her in a scam.
Sekine and Kazama were initially arrested in January 1995. In March 2001, a district court in Saitama Prefecture sentenced them to death for committing, in the words of the presiding judge, "cruelly ruthless and extremely heinous crimes."
The Tokyo High Court rejected the pair's appeal in July 2005, and the Supreme Court upheld the decision in June 2009.
