Iwamuto was born in
Miyoshi,
Hiroshima Prefecture. He took part in the
National High School Baseball Championship in 1931, but his team was defeated by a team of
Masao Yoshida (who was also later inducted to
Japanese Baseball Hall of Fame). He joined the
Nankai Baseball Club in 1938, but he was forced to take part in
World War II, so his professional debut didn't occur until 1940. The quality of baseballs was low due to the war, so it was difficult for hitters to hit home runs, but Iwamuto hit three home runs in one game on July 11, 1942, the Japanese record at that time. He retired once in 1942 and went to war again. Iwamoto returned to baseball in 1949, joining the
Taiyo Robins. On March 11, 1950, he hit the first home run in the history of the
Central League, a
grand slam home run. That same year, he hit three home runs in the first
Japan Series, which his team lost to the
Mainichi Orions, 4 games to 2. On August 1, 1951, he hit four home runs in one game, becoming the first professional hitter in Japan to achieve that feat. He retired again in 1953. He signed with
Toei Flyers and returned to the field again as a
playing manager in 1956. On August 18, 1957, he hit his final home run, at the age of 45, which is the Japanese age record. As manager of the
Toei Flyers from 1956 to 1960, Iwamoto compiled a record of 290–376, with 17 ties, for a winning percentage of .435. Iwamoto was inducted into the
Japanese Baseball Hall of Fame in 1981. ==References==