he amazing strength of Japan's economy is reflected in the prospect of a $15 billion trade surplus with the United States this year, despite "voluntary" quotas that have been placed on shipments of Japanese cars.

President Reagan's trade ambassador, William Brock, has been warning the Japanese government at top levels that such a surplus is "unacceptable," and his point has been driven home by a number of influential senators and congressmen who have given Prime Minister Zenko Suzuki something new to think about:

Unless Japan pitches in more money for global defense--giving up what the Americans like to call a "free ride"-- American protectionist sentiments will be deepened even further.

This trade-defense "link" has brought Japanese-American relations to a low ebb in an up-and-down cycle. Until now, the Japanese had assumed they had made "the supreme sacrifice" earlier this year by bowing to the Reagan-Brock demand for quota limits on cars, even though the Japanese feel that Detroit, not Tokyo, is to be blamed for the U.S. industry's disastrous performance.

The most significant development of all is that the alarm bells are being sounded by congressmen who are friendly to Japan and who basically are defenders of an open trading system. Their reactions in public and private meetings here in the past 10 days reveal an even more hostile attitude back home by those who perceive Japan's economic success as a combination of two things: excessive concentration on exports, and a closed door here to imports.

At the influential Shimoda conference at Oiso a few days back, one American congressman told a hushed audience: "What we are trying to say is that those of us here are committed to free trade--we are the ones that fought the auto quotas.

"But we are trying to tell you that you cannot feast off our market to the tune of $14 billion or $15 billion a year, year after year, and not get some sort of reaction. The Japanese never understand that--or if they do, they never admit it.

"No one has more to lose than the Japanese businessmen, who are living off export growth: If the American and European markets become more closed themselves, it will be the Japanese businessmen who lose out."

The Japanese point out that part of the problem has been the high interest- rate policy of the United States. This policy has pushed the dollar up and the Japanese yen down, thus making Japanese goods even more attractive in the American market. They also claim to have liberalized many of the barriers to imports of industrial goods, and complain that most American manufacturers show little initiative in breaking into the market here.

But there is a long way to go before it can be said that the Japanese have torn down all the barriers, especially on agricultural products. The main American argument is that Japan gives grudgingly, inch by inch, as it meets the pressure from Europe and the United States, never initiating a more open stance.

Thus, the perception remains among Westerners, even as they troop through the latest Japanese factories with nothing but open admiration, even awe, that some portion of the economic miracle is due to protectionism. And that being the case, the argument will be made in congresses and parliaments all over the world that to catch up, Japan's competitors must apply the same protectionist techniques.

Some thoughtful officials, such as Assistant Secretary of State Robert Hormats, fear that the problem will create difficulties for the international trading system as a whole in the 1980s, as protectionist and nationalist pressures become more pervasive.

For the moment, the huge Japanese bilateral surplus with the United States will lead to a demand for a "division of labor" between the United States and Japan on defense-security measures.

To help check the Soviet military machine, the Reagan administration is prepared to insist that Japan pay for the costs of defending its own islands and chip in more dollars to maintain American military forces in Japan. In addition, there is a shopping list being developed at the State Department of various economic measures that Japan could take to help stabilize the political balance in Asia.

Slowly Japan will be pushed more and more into military activities. It is easier for Japanese politicians to nudge defense spending upward than to fight their strong domestic farm lobby, or to respond to Western demands for penetration of the Japanese market.

Thus, one of the prices of economic success here may be the reemergence of Japan as a military power. Not too many years down the road, a top Japanese economist says privately, Japan's actual military budget may exceed that of West Germany.