A fuss of contentment is being made over the 30th anniversary of the U.S.-Japan Security Treaty, and a good thing too. The treaty is such an established part of the international landscape that people forget how well it has worn. It is right that it be celebrated and, beyond that, reviewed to make sure that the security tie between these two political allies and economic competitors continues to serve their needs.

In the United States, where the treaty is taken for granted, and in Japan, where it has been argued over from the start, the treaty is now seen as having brought Japan cheap and effective national protection and the United States a regional platform crucial to its chosen postwar global role. The current question is whether the changes in the Soviet Union that have produced the end of the Cold War dictate in Asia the same kinds of mutual military rollback and political relaxation that are coming to Europe.

The short answer is, maybe -- but not yet. The Soviet Union has barely begun to alter its Asian deployments. Communism remains installed in its other troubled Asian outposts. There is no Asian counterpart to either a democratizing East Europe or a reunifying Germany.

It follows that Japan should stick with its alliance with Washington in order to keep things steady and to deter threats from wherever. This is prettymuch the view of the Japanese government. It anticipates only prudent updating, not structural change: picking up more of the U.S. defense tab in Japan, some further return of land used for American bases, smoother ''mutual exchange'' of military technology and so forth.

In this spirit the Japanese government sent Shintaro Abe, an establishment heavyweight, to underline Tokyo's commitment to the traditional treaty bond. Japan now desires to be seen playing a more disinterested international role, not just flexing its economic might, he brought with him a proposal to create a large new fund in Japan to expand dialogue and exchanges.

Continuity, with suitable updating, is also the theme of the American government. Some of its specific rationales get blurry at the edges, but the core intent of sticking with a good thing, at no great cost or risk, is sound. ''Forward defense'' is the preferred American way to ensure deterrence, prevent a ''vacuum'' or ''destabilization,'' keep up with American trade and head off any possibility of resurgent Japanese militarism.

A delicate point: Japan's self-defense force costs only 1 percent of GNP but comprises the third-largest force in the world and could be greatly expanded. The embrace of the security treaty provides anxious Asians and the many anxious Japanese the necessary assurance that this force will stay within calming political and non-nuclear bounds.

What about, some ask, accepting Moscow as an interlocutor for an Asian reshuffle? Accustomed to geopolitical primacy in East Asia, the United States shies from accepting the Soviet Union as the ''Pacific power'' Mikhail Gorbachev has declared it to be. Washington has never granted Moscow Pacific parity. It particularly fears being drawn into an arms control negotiation in which the U.S. Navy would be put on the table and one-sidedly drawn down.

Still, Gorbachev has a card to play. He's due in Japan next year and may arrive ready to deal back the four of Japan's northern islands that Stalin took as World War II booty. This would soften much of the immense resentment Japanese feel toward Soviet estrangement of their national patrimony. It would fire up the nationalist faction that believes with Shintaro Ishihara, the Diet member who would sell Japan's chips to the Soviet Union, that the U.S.-Japan Treaty is ''losing its significance.''

This current in Japan could expect a certain reinforcement in the United States. We have both liberals and conservatives who, for their separate reasons, are eager for post-Cold War retrenchment in Asia. Others feel that a once useful but now overrated and anachronistic security connection mesmerizes Americans and keeps them from rolling up their sleeves and fighting the Japanese economic dragon.

These Japanese and American currents could yet be quickened by a Gorbachev diplomatic offensive in Japan and -- the real threat -- by an American protectionist backlash against Japan's trade and investment thrusts. This is something to keep a close eye on. But it is something to keep an eye on while understanding that the treaty retains value at both the Japanese and American ends.