© Agence France-PresseKanae Kijima
Kanae Kijima, 37, was convicted of murdering the three men, aged 41, 53 and 80, whom she met through Internet dating sites.
Kijima, who has a past as a paid-for mistress, killed the men by poisoning them with carbon monoxide from burning charcoal briquettes after giving them sleeping tablets.
Saitama district court heard how she had wanted to stop them from demanding back money she had taken from them over the course of brief relationships.
"Three times she carried out extremely serious and vicious crimes," presiding judge Kazuyuki Ohkuma said.
The thick-set Kijima, who according to media reports appeared in court wearing a short skirt, committed the crimes "in order to maintain her luxurious life full of vainglory without working", the judge said.
More than 1,300 people queued up for the 49 seats available to the public for the hearing.
The case gripped the public imagination and took on an extra dimension because, unusually, prosecutors achieved their conviction without the direct evidence Japanese courts often rely on, such as witness testimony or a confession.
Instead the prosecution rested on layers of circumstantial evidence, such as Kijima's purchases of sleeping pills and coal briquettes, in addition to the fact that she had met with each man shortly before he died.
She killed Takao Terada, 53, and Kenzo Ando, 80, in Tokyo and nearby Chiba prefecture in January and May of 2009, by leaving burning briquettes in their homes, a fairly common method of suicide in Japan.
Yoshiyuki Oide, 41, died in a rented car, also from the fumes from briquettes, in neighboring Saitama prefecture in August 2009.
Kijima's plain appearance seemed to commentators at odds with her defence that two of her victims had killed themselves because they were distraught at the idea of her leaving them.
She was also convicted of seven other lesser crimes, including fraud and theft.
Her legal team said she planned to appeal.
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