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,[citation needed] 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.

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.

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.