A 67-year-old millionairess has been arrested on suspicion of poisoning her husband with cyanide as it emerged six former partners had already died, in Japan's latest apparent "black widow" case.

Chisako Kakehi has been the beneficiary of a combined Y800 million ($A8.44 million) over the past two decades, Jiji Press said - insurance money and other assets she received after the seven men's deaths.

Chisako Kakehi, pictured here in March, was arrested this week on suspicion of poisoning her husband. Credit:Jiji Press/AFP

Husband number four Isao Kakehi fell sick suddenly at home and was confirmed dead at a hospital in December last year, less than two months after the couple married.

An autopsy found highly toxic cyanide compounds in his blood.