TOKYO, July 21 (Reuters) - Japanese Premier Shigeru Ishiba vowed to remain in his post on Monday after his ruling coalition suffered a bruising defeat in upper house elections, prompting some in his own party to doubt his leadership as the opposition weighed a no-confidence motion.

Ishiba's centre-right party has governed Japan almost continuously since 1955, albeit with frequent changes of leader. The result underscores voters' frustration with Ishiba, who has struggled to inspire confidence as Japan struggles against economic headwinds,