President Bush is naming Undersecretary of State Michael H. Armacost the new U.S. ambassador to Japan, where he will grapple with touchy trade and defense issues.

Armacost, 51, is the highest-ranking foreign service officer in the State Department. If confirmed by the Senate, he will succeed Mike Mansfield, a former Senate Democratic leader who was the ambassador for a record 11 years before retiring last November at age 85.Bush's selection of Armacost was confirmed Tuesday by a U.S. official after reports of the pending nomination circulated in Japan.

Three other diplomatic selections also surfaced: Bernard Aronson for the sensitive job of assistant secretary of state for inter-American affairs, Richard T. McCormack to be undersecretary of state for economic affairs and John Negroponte to be U.S. ambassador to Mexico.

Aronson is a Democrat who broke with the majority of his party to support U.S. aid to the Nicaraguan reb-els. McCormack is currently assigned to the inter-American bureau. Negroponte, deputy national security adviser, was a candidate to succeed Armacost.

Armacost is a former U.S. ambassador to the Philippines and was the ranking Asian specialist on the National Security Council staff in 1978-1979, during the Carter administration.

He has held a Pentagon post as well, serving as deputy assistant secretary of defense in 1979-80.

Under former Secretary of State George P. Shultz, Armacost played a key role in dealing with Soviet authorities on regional disputes in Afghanistan, southwest Africa, Cambodia and Central America.

And Armacost, in a memorandum to Shultz on July 2, 1986, counseled him to oppose President Reagan's plan to send U.S. missiles to Iran in an effort to free American hostages.