Unsolved problems between Japan and the United States could lead to dangerous confrontations next year, says United States Ambassador Mike Mans-field. Tensions will rise, he says, as protectionist sentiments pick up during the presidential and congressional campaigns in the United States.
In a meeting with American journalists here Wednesday, Mr. Mansfield applauded Japan's efforts in the defense field but said that on trade, Tokyo would have to ''flesh out'' packages it has worked out to increase imports and reduce friction with its major partners.
The ambassador believed that Japanese officials were thinking in terms of some kind of specific friction-easing steps to be taken before President Reagan's visit to Tokyo in November. But he did not elaborate.
On defense, the Nakasone Cabinet has just reached a compromise decision calling for a 6.9 percent ceiling on increases in Japan's defense budget for the coming fiscal year. The ceiling is a theoretical figure beyond which the actual budget will not be allowed to go. During the current year, for instance, the ceiling was 7.3 percent and the actual increase was 6.5 percent.
Mr. Mansfield said he was impressed with this because the figure showed continuation of the ''steady and significant progress'' Japan has made in defense during the past 13 years. In dollar terms, if the entire ceiling is actually appropriated, there would be a $789.6 million increase over this year's
Mr. Mansfield said that, in NATO terms, Japan is already spending about 1.5 percent of gross national product for defense, rather than the 1 percent usually cited. This is because in Japan certain welfare and pension expenditures, which the United States and many other NATO countries include in their defense budget, are put into the welfare budget.
Defense has become an extremely controversial subject in Japan. Government deficits are piling up, yet Prime Minister Yasuhiro Nakasone's views on the Soviet threat and need for improved defense are probably closer to those of the US than were any previous Japanese premier's. He has called for drastic pruning of every major budget item except defense and foreign aid.
In the budget being prepared for next year, the Finance Ministry has set guidelines of a 10 percent decrease in the general budget, as compared with last year, and a 5 percent decrease in the investment budget. In practice, because a number of expense items automatically rise, some growth is inevitable. However, the general budget next year will rise by only 1.0 percent - unprecedented since the practice of annual ceilings was begun in 1961.
A 6.88 percent increase in defense spending under these circumstances is therefore exceptional, and this is what Ambassador Mansfield chose to praise.
But the Pentagon, which has been calling for much larger increases in the Japanese defense budget, is unlikely to be pleased. The ambassador himself conceded that it probably would not be possible for Japan to take over principal responsibility for its own defense until some time near the end of this century. Until then, Japan relies principally on the US under a security treaty whereby Washington helps defend Japan and Japan provides bases for American armed forces.
On the trade issue, Mr. Mansfield suggested Premier Nakasone was ''racking his brains'' for ways to put flesh on the successive trade packages designed to ease friction with the US and other major partners. He noted that, for all the friction over beef and citrus fruits, Japan was already buying 60 percent of all American beef exports and 40 percent of all American citrus exports.
On cars, Mr. Mansfield said International Trade and Industry Minister Sosuke Uno had probably been misunderstood when he recently said Japan would not continue voluntary restrictions on US car exports next year. Japan has continued these restrictions for three years, and Mr. Uno has not foreclosed the possibility of renewing them.
Mr. Mansfield, former Democratic majority leader in the Senate, has been an immensely popular and respected ambassador to Japan since his arrival in early 1977. He is one of the few diplomats from the Carter administration still at his post. A hale, hearty, and vigorous octogenarian, Mr. Mansfield said he intended to remain in his job ''indefinitely'' after a brief holiday in the US during the summer.