Mike Mansfield announced his retirement Monday after 11 years as U.S. ambassador to Japan and a public service career that began at age 14, when he lied about his age to enter the Navy during World War I. The 85-year-old former Senate majority leader has worked tirelessly for strong U.S.-Japanese relations, which he described as excellent, with most problems resolved. He said he would leave his post before Jan. 1. Mansfield said he and his wife, Maureen, made the decision together. ``We decided it was time for me to resign, subject to the will of the president, and that has been done,'' the Democrat from Montana told reporters at the U.S. Embassy. He said they would divide their time between Washington and his home state, spending winters in Florida. Senior Japanese officials expressed appreciation for Mansfield's years of effort for improved relations. ``It is very regrettable that Ambassador Mansfield is resigning,'' aides quoted Foreign Minister Sosuke Uno as saying. ``He was a good friend who understands Japan quite well. ``It was truly fortunate that we had a person like Ambassador Mansfield in Tokyo for a long time. ... His warm personality has earned everyone's respect and love.'' Mansfield set records for length of service with 16 years as Senate majority leader and 11 as ambassador to Japan. The Japanese welcomed his appointment to the Tokyo mission by President Jimmy Carter because of Mansfield's prestige in Congress and his belief in the importance of East Asia to the United States. He has called the next century the ``century of the Pacific basin'' and often said the relationship with Japan is the most important to Washington, ``bar none.'' Speculation arose that Mansfield would retire after coronary bypass surgery in January, but he returned to work in the spring. The ambassador said Monday he and his wife agreed at the time to delay retirement until after major trade issues were resolved and the U.S. presidential election was over. The recent U.S. decision, with Mansfield's support, against trying to force the opening of the Japanese rice market at this time helped clear the way for his retirement, he said. Trade problems are the major cause of friction between the United States and Japan but relations generally are in ``excellent shape,'' Mansfield said. `In the field of politics and diplomacy we work very well together,'' he told the news conference. ``In the field of security, the relationship couldn't be better, and in the area of investment things are going very nicely.'' Some people both in and outside the U.S. government have criticized Mansfield as being too accommodating toward the Japanese. He says in response that Japanese markets have opened in many ways and Japan has assumed a significant part of the security burden in Asia. He advocates a broad trade agreement to ease friction and the idea is being studied, but is unlikely to materialize soon. Mansfield said Monday he treated the Japanese as he would wish to be treated and they responded in kind. he called the Tokyo years ``the most enjoyable in our lives.'' After beginning seven decades of service to the United States with the Navy enlistment at 14, Mansfield served in the Army and the Marine Corps. He was in Asia with the Marines. Before his election to Congress in 1942, he taught Far Eastern and Latin American history at Montana State University in Missoula. The school since has been renamed the University of Montana. In 1944, Mansfield was sent on a foreign policy mission to China at the request of President Franklin D. Roosevelt. Before his appointment to Tokyo, he had returned to China several times at the Chinese government's invitation.