WITH THE ELECTION of Yasuhiro Nakasone as prime minister, a long period of indecision at the top of the Japanese government seems to be ending. His predecessor, Zenko Suzuki, seemed to regard himself as an interim figure, filling the job while the ruling Liberal Democratic Party debated its next steps. Once again, in the tradition of the party, the question of leadership has now been resolved essentially on personal grounds, with policy positions to come later.

Mr. Nakasone arrives with a record of promises that will sound very familiar to Americans. He wants to reduce the size and cost of government. He has been a supporter of a stronger defense force. But as a practical matter, present Japanese policy on public spending and defense seems unlikely to change much. The real business of the new government lies elsewhere.

No country has a larger stake than Japan in the international system of trade, and that system of trade is now in some considerable peril. The current conference of GATT -- the General Agreement on Tariffs and Trade -- in Geneva is degenerating into a sullen exchange of accusations. The Japanese government never engages in this kind of recrimination, but, on the other hand, it rarely does much to alleviate the fears and political tensions that underlie it in other countries.

The superior Japanese performance in world exports is based on superior products and superior sales organizations. But the extraordinary increase in the Japanese companies' shares of foreign markets in the midst of a deep recession has contributed much to bringing the whole principle of open trade into jeopardy. The Japanese government, at least up to this point, has been reluctant to acknowledge that it has any responsibility to do anything about it -- or even that there is any real substance to other people's complaints.

One example is the exchange rate of the yen, which has slid way down. The Japanese government didn't engineer it, but neither did it do anything to prevent it. The effect has been to give Japanese goods, already highly competitive, a further and ferocious advantage in foreign markets. Over the past several weeks, the yen has risen significantly, but it is going to have to rise a lot farther before it reaches the range that most people consider its true trading value. If Japan's government continues to treat the yen rate as none of its business, and the rate stays low, pressures will continue to rise here and in Europe for the most pernicious and destructive kinds of protectionist legislation.

It is disquieting that nothing audible from Tokyo suggests that the incoming prime minister gives any particular priority to foreign economic policy and the maintenance of trading relationships. But they have become crucial to standards of living in all the industrial countries. A great deal depends on Mr. Nakasone's attention to them -- a great deal not only for the Japanese but for Europeans and Americans as well.