PRESIDENT Reagan's long lunch with Yasuhiro Nakasone, the prime minister of Japan, was evidently a very pleasant and relaxed affair. A number of people in the administration are currently very irritated with Japan, but the president is not one of them. They are mostly trade specialists who accuse the Japanese of resorting to unfair restraints to hold down their imports from the United States. The trade specialists in any administration are always uneasy about this tradition of warm meetings between the heads of the two governments. In talks at that level, the trade quarrels are always balanced -- wisely -- against the strategic and political interests that the two countries share. Approached that way, the trade agenda usually seems a little less urgent. A succession of presidents has chosen not to press the Japanese as hard, personally, as their trade negotiators would have liked -- not to mention the American exporters who egg the negotiators on.

After their meeting this week, Mr. Reagan and Mr. Nakasone said that they would set up discussions of ways to open Japanese markets wider. That's a reasonable idea, but hardly a new one. The American government has been complaining for years that the trade between the two countries is unbalanced in Japan's favor. Periodically the negotiators sit down and, in time, produce a series of measures that are supposed to open up the Japanese market and put things right. But, when the same people sit down again a year later to talk about the same complaints, the imbalance is usually bigger than ever.

Perhaps it will be different this time. But there are two factors -- one on each side -- that will limit the success of even the most vigorous and earnest efforts to increase American sales to Japan. On the Japanese side, the barriers to imports these days are not the kind of legal quotas or regulations that a government can cancel. The real barriers are attitudes -- the cautious Japanese inclination to prefer Japanese products and to avoid becoming dependent on any foreign source of supply in any but the utterly unavoidable cases, such as industrial raw materials. Opening up the market to manufactured imports takes not government decisions but an extraordinary amount of salesmanship and cajoling.

On the American side, there is the reality that the U.S. dollar's exchange rate is now extremely high. Against the yen, in terms of the things that it can buy, it is now overvalued by about one-fourth. A Japanese buyer has to want an American product badly enough to pay a 25 percent premium over the price of its Japanese competitor -- or, for that matter, its French or German competitor. As long as American fiscal mismanagement perpetuates an overpriced dollar, the prospects of reducing trade deficits through exhortation will be, at best, dim.