Former Senate majority leader Howard Baker announced on Friday that he was ending his tenure as U.S. ambassador to Japan and would leave the country in February.
Tom Schieffer, the U.S. ambassador to Australia, is considered the most likely replacement.
Baker, 79, had a long career in politics before being named ambassador by President George W. Bush in 2001. He was a three-term Republican senator from Tennessee, Senate majority leader, presidential candidate and White House chief of staff to Ronald Reagan.
Baker had open heart surgery in August and returned to Tokyo in October.