Mizuho Financial Group said on Monday its net profit more than tripled in the nine months ended in December, buoyed by hefty bond trading gains in the first half, and kept its full-year profit outlook unchanged.

Japan's second-largest bank by assets said net profit came in at 422.1 billion yen ($5.1 billion) for the April-December period, up from 126.3 billion yen a year earlier.

For the first six months of the current financial year, Mizuho and its rivals saw their earnings boosted by sharp rises in prices of U.S. Treasury and Japanese government bonds, although they started falling late last year.

For the year ending in March, the bank retained its net profit forecast of 500 billion yen, up from 239.4 billion yen a year earlier and in line with an average estimate of 507.3 billion yen in a poll of 14 analysts by Thomson Reuters I/B/E/S.

($1=82.10 Yen)

(Reporting by Taiga Uranaka; Editing by Muralikumar Anantharaman and Edmund Klamann)