"The terms of article eight (of the International Convention for the Regulation of Whaling) are unambiguous," said Professor Pellet, of the University of Paris Oeust.
"The decision-making power rests with the state party concerned," he said. "Not to be a subject of multi-lateral decision-making...This is a discretionary power that article eight grants to state parties."
Professor Pellet was responding in The Hague to Australia's plea for the ICJ to halt the program, as disguised commercial whaling that has killed more than 10,000 whales.
Japan replied this week alleging that Australia was on an alarmist crusade intended to impose Australian cultural preferences over the Japanese.
On Wednesday the Japanese legal team moved to broaden its defence before the court.
Professor Shotaro Hamamoto described the current Japanese program, known as JARPA II, as the most comprehensive research program ever carried out on whales and the Antarctic ecosystem.
It had concrete research objectives, and gained support from the International Whaling Commission's scientific committee, particularly in a 1997 review, said Professor Hamamoto, of Kyoto University.