KYOTO - A so-called "black widow" accused of killing three partners and attempting to murder a fourth admitted on Monday (July 10) to fatally poisoning her husband.

Chisako Kakehi, 70, took the stand for the first time at the Kyoto District Court, local media reported, with prosecutors alleging she had committed the crimes between 2007 and 2013.

"I killed my husband," said Kakehi, whose trial started on June 26.

Kakehi's husband, Isao Kakehi, died in December 2013 at the age of 75. A cyanide compound was detected in his body, according to The Japan Times.

She reportedly tricked him into taking the poison, which she obtained from a business associate, and likely put it into a health food capsule.

When asked why she had committed the murder, Kakehi admitted she had killed him for his money as she thought she would be able to repay her debts if he died.

She claimed that he treated her poorly - especially in financial matters - and that she was not given any money after marrying him.

Kakehi's lawyers have asserted her innocence, arguing that she cannot be held liable for her crimes as she has mild dementia.

The Japanese press have dubbed her a "black widow", after the female spider that eats its partner after mating.

Besides Mr Kakehi, she is also on trial for murdering two other common-law husbands and the attempted murder and robbery of another boyfriend, who later died of cancer.

All four men were aged between 70 and 80.

The Japan Times, quoting investigative sources, said more than 10 men romantically involved with her are known to have died. She has inherited an estimated 1 billion yen (S$12.2 million).

In April, another "black widow", 42-year-old Kanae Kijima, lost her final appeal against her death sentence for murdering three boyfriends she met online and dated for their money.