After serving as a town council member in Shiga, Iwanaga was elected to the House of Representatives in 1996, representing the fourth district of Shiga. He joined the Kato faction of the Liberal Democratic Party, but later switched to the Horiuchi faction. He became Vice Minister of Agriculture under the first Koizumi cabinet.[1]
On August 11, 2005, Iwanaga became Minister of Agriculture following the resignation of Yoshinobu Shimamura.