Motoo Noguchi was named by President Mahinda Rajapaksa on the advisory panel of the Presidential Commission investigating the cases of disappearances.
Noguchi, who would be the sixth member on the panel, is a former international judge of the Extraordinary Chambers in the Courts of Cambodia, and the Chair of the Board of Directors of the Trust Fund for Victims of the International Criminal Court.
Previously, five international experts have been appointed to advise the commission: Sir Desmond de Silva and Sir Geoffrey Nice from the UK, Prof David Crane from the US, India's Avdhash Kaushal and Ahmed Bilal Soofi of Pakistan.
The Tamil Tigers were were defeated after their leader Velupillai Prabhakaran was killed by the forces in 2009.
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The commission has received nearly 20,000 complaints of disappearances so far. Over 5,000 of them have come from the relatives of security forces personnel.
The commission has held public sittings in the former battle raged areas of Killinochchi, Jaffna, Mullaitivu and Mannar in the north and Batticaloa in the east beginning this year. Tamil families affected by the armed conflict appeared before them.
In July, Rajapaksa extended the mandate of the commission to probe any violations of international humanitarian and human rights laws.
This was to be Sri Lanka's own domestic mechanism in the wake of the UN Human Rights Commission initiating an international investigation into alleged war crimes commited during the separatist conflict.