Iwamasa was born in Suo-Oshima, Yamaguchi on January 30, 1982. He was educated at and played for Iwakuni High School and Tokyo Gakugei University before turning professional. He started his professional career at Kashima Antlers where he later became one of the club's longest serving players. He became a regular player at center back from late first season. The club won the champions in J1 League for 3 years in a row (2007-2009) first time in J1 League history. He was also selected Best Eleven for 3 years in a row. The club also won 2007, 2010 Emperor's Cup, 2011 and 2012 J.League Cup. His opportunity to play decreased in 2013 and he left the club end of the 2013 season.