JAPAN'S new Prime Minister Tsotumo Hata has enough problems keeping his minority government in power without having to deal with senior cabinet ministers whose first act in office is to insult the memories of millions of Asian victims of his country's imperialist past. Nevertheless, Mr Hata should have no hesitation in sacking his Justice Minister Shigeto Nagano for claiming the Nanjing Massacre was a hoax. Such revisionism cannot be tolerated in the government of a country which hopes to play a leading role in the affairs of the region, retain normal diplomatic relations with its Asian neighbours, or hold its head high in the international community. Nor should it be acceptable domestically. It is extraordinary that a man who has not only served as his country's military Chief of Staff, but aligned himself politically with a coalition which favours clean government, political reform and a modicum of honesty about the nation's past, should have the arrogance to make so light of a massacre in a neighbouring country. At least 155,000 and possibly up to 300,000 died in the rape of Nanjing. The most damning aspect of Mr Nagano's behaviour, however, is that it is not only acceptable but praiseworthy to a worryingly large section of Japanese opinion. Among those who are keenest to see Japan change its pacifist constitution and contribute to international defence are an element on the Japanese right wing who claim the country's role in World War II was not aggressive. This is a tendency which must be kept under the firmest control. Mr Hata may represent that portion of Japanese opinion which believes Japan should take part in peace-keeping, not war-mongering. But if there is a tendency within his ranks towards a more aggressive stance he should perhaps reconsider. The Socialists who left his coalition government last month did so partly to preserve their commitment to the post-war pacifist constitution. If the partnership they walked out on includes many more like Mr Nagano, it is clear they had a stronger point than the West has given them credit for. For as long as Mr Hata hesitates over sacking Mr Nagano, the Socialists and Japan's Asian neighbours will have a right to be nervous. Delay should also give pause to those like French President Francois Mitterrand, whom Mr Hata thanked yesterday for his support for a permanent Japanese seat on the UN Security Council. If Mr Nagano can be permitted to make such statements from within the Japanese cabinet, the world will have every reason to doubt whether Japan has the political maturity for such a responsible international role.