THE UNITED STATES has just won a notable victory in the politics of world trade. Japan has agreed to let American companies bid when the government's Nippon Telephone and Telegraph Corporation buys equipment. As in all trade victories, no one will know for years exactly how important it might be. That depends on the spirit in which Japan carries out its promise and the skill with which the American electronics companies develop this new opportunity. But if Japan had not opened this market to foreign bidders, the result would have been a damaging defeat not for the United States, but for the expansion of trade and the prosperity that trade brings.

In the early 1970s, trade was expanding rapidly around the world. To keep the process going, most of the world's governments joined to reduce the barriers by which countries protect various favored and influential industries. These trade negotiations became known as the Tokyo Round, and went on for five arduous years. One provision in the final agreement urges governments, when they buy, to give foreigners the same standing as their domestic suppliers. That provision goes into effect on Jan. 1 and, if Japan had refused to open up the Nippon Telephone bidding, the United States could have legally retaliated by barring all Japanese companies from any government sales in this country.

The Japanese argued and resisted. But the Nippon Telephone rule was clearly protectionist, and on Friday, two weeks before the deadline, the Japanese dropped it. The intricate machinery built in the Tokyo Round worked successfully. Like any modern industrial country, the Japanese have far too much at stake in their exports to risk retaliation against them. Precisely as it was intended to do, the Tokyo Round system forced a government to choose between one industry's special interests and the broader requirements of the whole economy.

Some of Mr. Reagan's advisers are inclined to abolish the office of the president's Special Trade Representative, which negotiated both the Tokyo agreements and the Nippon Telephone settlement. Under one current proposal, the office's work would be relegated to the Commerce Department. That would not be a trivial change. The loudest and most aggressive part of the Commerce Department's constituency consists of those American companies that want government help -- which, in trade cases, means protection from their foreign competitors.

Mr. Reagan, who has been firmly in favor of open and competitive markets, will find that his policy will be move reliably followed if he keeps the trade talks attached directly to the White House. It's not only consumers who benefit from the principle of open markets. As the Nippon Telephone case demonstrates, it can bring important new opportunities to American producers as well.