Born in Hyogo Prefecture,[2] Ienaga began his career as a trainee with Gamba Osaka, one of the leading clubs in Japan's J1 League.
[3] Plymouth attempted to sign the player in January 2009 but he was denied a work permit so they were forced to withdraw from talks.
[4] In December 2010, he signed with Spanish Primera Division club RCD Mallorca in a five-year deal.
[6] In June 2005, Ienaga was selected Japan U-20 national team for 2005 World Youth Championship.
Ienaga made his international debut as a substitute in Japan's 2–0 win against Peru in March 2007.