JAPAN'S NEW new prime minister, Yasuhiro Nakasone, is about to visit the United States, and to smooth his welcome he has made a few policy adjustments. He cut duties on a range of farm and industrial products and put through a defense increase. We have addressed trade on other occasions, and here we look at defense.

Unfortunately, there's not much to look at. The latest increase comes to about 4 percent in real terms and keeps defense still below 1 percent of GNP. The increment leaves Japan well behind its own five-year procurement goals and puts off to the next century the time when Japan would be able to carry out the air defense and sea-lane defense missions it has formally agreed to. In brief, the Japanese, with growth at 3 percent, inflation at 3 percent and unemployment at 2.4 percent, are again validating the "free rider" label they find so displeasing and unfair.

It is sometimes said that Americans overlook the contributions Japan makes to the common defense and that they fail to appreciate the special historical and political considerations rendering a greater Japanese effort unfeasible and unwise. To us, however, it seems truer to say that no country has enjoyed a more sympathetic hearing in this country, for longer, than Japan. Precisely this high standing seems to have led a succession of Japanese governments to believe they could safely ignore the warnings on defense and trade that have come from both major political parties in the United States and from both branches of government.

As one friendly critic, Rep. Stephen Solarz, has noted, it is not that the United States is urging a major rearmament program, renegotiation of the U.S.-Japan security treaty or revision of the Japanese constitution. 'All the United States asks," he writes in Foreign Policy magazine, "is that Japan acquire the military capabilities necessary to defend its territories and to resist Soviet pressure and threats--in cooperation with remaining U.S. forces. . . . (T)he United States is unable to defend Japan without Japanese assistance." Mr. Solarz thinks Japan could spend 2 percent of GNP on defense.

The Japanese have been unusually shortsighted in failing to keep their trade with the United States more in balance. The same insensitivity is apparent in defense. Trade frictions and defense frictions are now growing and feeding each other. We think this is the situation Mr. Nakasone will find on his visit, and we hope it is the message he takes home.