But the property developer was cleared of killing Lucie Blackman, a former flight attendant from Kent who was working as a hostess in a Tokyo nightclub when she disappeared in July 2000 aged 21.
Speaking at a news conference after the ruling at Tokyo district court, Miss Blackman's father Tim Blackman and her sister Sophie, 26, spoke of how they felt "completely unresolved" at the decision.
"When we came out of the courtroom we were quite pragmatic about the sentence, we felt that Obara had received a just sentence," Mr Blackman said.
"But following our meeting with the prosecutors we feel much more upset about it and feel that in many respects Lucie has been robbed of her justice."
The young Briton's dismembered body was found in a cave in the remote fishing village of Miura outside Tokyo, close to Obara's home, seven months after she had disappeared.
The prosecution alleged that Obara drugged and raped Miss Blackman before dismembering her body and encasing her head in concrete – charges continuously denied by the businessman who was already in custody on suspicion of a number of rapes and the murder of another nightclub hostess when first questioned about Miss Blackman's disappearance.
In court today, judge Tsutomu Tochigi said of Obara: "There is nothing to prove that he was involved in the rape and her death. The court cannot prove he was single-handedly involved in her death.
"What is clear is that the victim acted together with the accused and then vanished and, following that, she was found dead."
But, speaking at the same conference alongside her father, Sophie Blackman cast doubt on the verdict.
"The evidence that definitely linked Obara to Lucie's death, in our opinion, was not used," she said.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.