Matt Searle, from the Lucie Blackman Trust, said he had been told Obara had been convicted of eight rapes and one count over the rape and death of an Australian woman, Carita Ridgway.
The verdict comes as Japanese police hunt the killer of a British woman whose body was found in a sand-filled bathtub at an apartment outside Tokyo last month.
Meanwhile, it was reported that Carita Ridgway's family is calling for an independent inquiry into Tokyo Police's handling of the investigation into her death.
In a statement, the family said: "It is imperative there should be an inquiry into the lack of action on the part of the Japanese police in 1992, and also in 2000, when Lucie Blackman went missing.
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