Iwamoto was born in Yokohama on May 2, 1972. After graduating from Yokohama College of Commerce High School, he joined Fujita Industries (later "Bellmare Hiratsuka") in 1991. In 1994, the club won 1994 Emperor's Cup. In Asia, the club also won 1995 Asian Cup Winners' Cup. After that, he could not become regular player due to injuries. In the latter the 1990s, he played for Kyoto Purple Sanga (1998), Kawasaki Frontale (1999) and Verdy Kawasaki (2000). In 2001, he moved to Vegalta Sendai. At Vegalta, he became a regular player and assisted many goals. In 2004, he moved to Nagoya Grampus Eight. However he got hurt in May and he left the club end of the season. In October 2006, he signed with Auckland City and played until December. In December, he also played at 2006 Club World Cup in Japan. He retired after 2006 Club World Cup.