After Mansfield san

ONE of the most important ambassadorial appointments George Bush will have to make is that of a new envoy to Tokyo. The redoubtable Mike Mansfield, longest-serving US ambassador to Japan in history, has announced he will step down after more than 11 years and return to the United States at the end of the year.

He has been known as a staunch friend of Japan - sometimes, by American lights, a bit too staunch. He has praised the Japanese for their maturing in the family of nations, for their progress in opening their markets to imported goods, and for their assumption of a larger share than is realized of the burden of defense in the Pacific.

But however appreciative the two nations are for the service of a faithful friend, this very important international relationship must rest on a broader base than that provided by single individuals - as we have no doubt Mansfield san, as the Japanese call him, would agree. And much as we admire a diplomat's professional optimism and willingness to give credit where due, the fact is that a long agenda of US-Japanese issues remains; the new envoy's work will be already cut out. Overwhelmingly, the US-Japanese agenda is one of trade issues; agriculture and services, especially financial services, are top priorities.

Opening Japanese markets to American agricultural products is important, because of the American farm lobby's political strength but also because agricultural products are one of the relatively few types of goods Americans can sell the Japanese at a good price. An agreement - ``sort of,'' as one US economist puts it - has been reached to open the Japanese market to American beef. But the agreement can't yet be said to be fully in place. And one response of the Japanese to demands to open up to imported beef is that they themselves are becoming producers of American and Australian beef, buying up ranches now as well as office towers.

A citrus agreement exists as well, though it likewise must be acted upon. Americans have, sensibly, backed off on the rice issue, partly for cultural reasons - Japanese still worry about having adequate rice in case of another war - and partly for logistical reasons. If the Japanese wanted foreign rice, they could buy from Thailand.

There has been progress on the financial services agenda, but US brokerages and banks are still not allowed into the Japanese securities industry on an equal footing with the local competition.

Another issue is access of American goods to the distribution system, the network of mom-and-pop retail shops that is effectively one of Japan's principal guaranteed-employment programs.

These issues are not insoluble, and indeed, they must be resolved if the US is to bring its trade deficit under control. But they will take years of patient negotiation. The new ambassador should be someone who can settle in for a long time.