JAPAN has become one of the last rich nations to lift economic sanctions against South Africa. But it may soon become that country's No. 1 trading partner, providing an economic boost to the white-run government in Pretoria.Just five years ago, Japan was the top trader with South Africa, triggering resentment among black African nations that nearly cost Japan a seat on the United Nations Security Council. This year, Japan waited until after it won another seat on the UN Security Council before announcing Oct. 22 that it would end most sanctions. The government ended a ban on loans and investment and now allows imports of South African gold coins. It justified the end of sanctions on the assumption that the reform process to end white-rule in South Africa is "irreversible," according to government spokesman Misoji Sakamoto. But Japan's delay in lifting sanctions several months after similar moves by the United States and the European Community, was widely recognized by African diplomats in Tokyo as a political maneuver. They knew Japan's top six steel makers had already signed a new five-year contract starting last April to import iron ore from South Africa. And in April and July, two groups of top Japanese businessmen visited the country to search for new investment opportunities. Japanese car makers seek a stable trade with South Africa in order to obtain platinum, a key element in the catalytic converters of exhaust systems. South Africa was also previously a big market for Japanese-made cars. Japan has imposed various sanctions against Pretoria since 1969. When it was the leading trade partner in 1986 and came under sharp criticism from African nations, Japan moved to freeze trade and banned official exchanges of people. It also slowed down car exports in 1988. Last June, the people exchanges were resumed when the last legal pillar of apartheid was removed. A similar move was made Oct. 18 by Commonwealth nations, which agreed to phase out sanctions. Japan also lifted its restrictions on loans to and investment in South Africa and will discontinue its voluntary ban on importing gold coins made in South Africa. When Nelson Mandela visited Japan a year ago and spoke before parliament, he was politely received but rebuffed by the government after requesting $25 million for his African National Congress. The ANC has opposed lifting sanctions until white-rule is ended in South Africa. Japan did not lift a ban on exports of arms or computers. Also, it decided not to exchange ambassadors. Still, the government is considering inviting South African President Frederik de Klerk to visit Japan. Finance Minister Barend du Plessis was in Tokyo recently "on a private visit."