JAPAN IS A difficult case. It sells far more in the United States than it buys here, and the long quarrel over the trade imbalance continues, with tempers rising. Americans have trouble selling in Japan for reasons that sometimes are as simple as quotas and tariffs. But much more commonly the resistance is deeper -- an engrained Japanese attitude that it's not quite right to buy foreign goods. A few American companies have managed to overcome that nativist resistance and establish themselves in the Japanese landscape, but not very many. Should the Japanese buy more American products? Yes, certainly -- for the sake of their own standard of living, and their government has a responsibility to lead them more forcefully toward open trade.

But there's a little more to the subject than that. Underneath all the American irritation with Japan -- and it's real -- there is a bit of guilty uneasiness. In matters of money and economics, the Japanese are behaving in the way Americans keep saying they themselves should be behaving. The Japanese save heavily, as Americans know they should but do not. The Japanese invest heavily, as Americans also know they should. The Japanese export their surplus savings abroad, as Americans used to before the administration changed the rules and turned this country into the world's biggest borrower. Most Americans know perfectly well that they shouldn't live so heavily on credit. It does not help their relations with Japan to be reminded that the Japanese are currently the world's great example of the Puritan ethic.

Japanese send their savings in very large amounts to the United States, where they help finance the Reagan administration's budget deficits. As the money moves from one side of the Pacific to the other, from yen into dollars, it pushes the exchange rate of the yen down and the dollar up. Because of those enormous financial flows, the dollar's exchange rate against the yen is now about 30 percent higher than its worth in American goods.

For the Japanese exporter, it means that every sale for dollars brings a 30 percent rebate. For the Americans trying to sell their products in Japan, the exchange rate is like a 30 percent tax. Is it surprising that most of the trade is westbound?

Even with this huge disadvantage, American exporters manage to sell quite a lot in Japan -- some $22 billion worth last year, more than to any other country but Canada. It's unlikely that sales will stay that high, let alone rise, if the exchange rate continues at the present level.

Up to a point, it's not unfair to chide Japan for its far from open market. But Americans might usefully keep it in mind that the greatest barrier to a better trade balance is not in Japan. It's here in the UnitedStates, where a reckless economic policy and excessive borrowing are putting an impossible burden on American export industries.