U.S. Ambassador to Japan Mike Mansfield announced his retirement Monday after nearly half a century of public service, including 16 years as Senate majority leader from Montana.

"I will resign subject to the will of the president," said Mansfield, 85, who underwent a successful heart bypass operation in January in Washington.Mansfield said he felt it was time to leave Japan with President-elect George Bush coming into office and U.S.-Japanese relations on solid ground.

"We can leave with our heads held high and our arms swinging," the lean, angular Montana native told reporters at a news conference at the U.S. Embassy he headed for 11 years. The diplomat said he was not asked to leave and had no idea who would be selected to replace him.

Mansfield, a Democrat for 34 years in Congress, including a record tenure as Senate majority leader, was appointed ambassador to Japan by President Jimmy Carter in 1977.

President Reagan confirmed Mansfield's value as America's representative to Tokyo when he asked Mansfield to stay on under the Republican administration.

Mansfield's 11 years in Tokyo made him the longest-serving ambassador to Japan since a U.S. Embassy was established at the end of the Russo-Japanese War in 1905.

Mansfield said he plans to return to the United States before the new year and live in "Montana any time but winter" and in Florida during the cold weather.

True to his humble style, Mansfield, whose career spans eight presidencies, said he has no intention of writing his memoirs. "I've noticed that people who write those books never make mistakes," he said. "It's always somebody else who made the mistakes."

The Japanese liked Mansfield because he often took an edge out of Japan bashing in Washington. Prime Minister Noboru Takeshita praised Mansfield, saying he is deeply grateful for his long years of devotion to better relations.

"He was a good friend of Japan who understood the country well," said Foreign Minister Sosuke Uno.