Japanese Ambassador Nobuo Matsunaga says a free trade agreement between his country and the United States is being considered more seriously because recent successes in resolving trade disputes have improved relations between the two nations. It is time to ``build our partnership on the basis of our interlocking strengths instead of constantly warring over our separate weaknesses,'' Matsunaga told the Washington Council on International Trade last week. The ambassador said that the much-discussed U.S. trade deficit and the corresponding Japanese trade surplus ``simply do not comprehend'' the scope and complexity of the economic and political relationships between the two countries. ``It is quite understandable, therefore, that thoughtful people on both sides of the Pacific are now considering the possible benefits of some form of free trade arrangement between the United States and Japan, the two largest and most dynamic economies in the world, whose two-way trade is exceeded in value only by America's trade with Canada...'' Matsunaga said in his Thursday address. A U.S.-Japan free trade agreement _ which essentially would remove trade barriers between the two nations _ was proposed first in 1981 by Mike Mansfield, U.S. ambassador to Japan. The initial reaction was widespread skepticism that two countries with such different cultures and legal systems could fashion a workable agreement. But earlier this year, Senate Majority Leader Robert Byrd raised the issue with Japanese Prime Minister Noboru Takeshita. And Sen. Lloyd Bentsen, chairman of the Foreign Relations Committee and now Democratic vice presidential nominee, asked the U.S. International Trade Commission to prepare a report listing the pros and cons of such an agreement. The interest is driven not only by the increasingly complex trade relationship between the two countries, but by concerns by both about their trade relations with other nations. Japan has concerns about how the U.S.-Canada free trade agreement may affect its own trade with the United States and Canada. And both countries are concerned that the lowering of trade barriers among European Community nations, scheduled for 1992, could lead to increased difficulty in gaining access to that market. David Rohr, a member of the federal International Trade Commission who recently visited Japan to gather opinions for the study requested by Bentsen, said the Japanese are ``really scared'' of the sweeping U.S. trade bill _ signed into law last week and criticized by Matsunaga as ``disproportionately protectionist'' _ because of its sanctions for unfair trade practices.