A native of Nirasaki, Yamanashi, and graduate of Tsuru University, he was elected to the House of Representatives of Japan in 1990, where he served for two terms until 1996 when he failed to be re-elected.

[3] In January 2016 he announced that he would not seek a fourth term and was replaced by Democratic Party candidate Yuka Miyazawa [ja] at the July 2016 election.

[4] His appointment happened as Yoshihiko Noda replaced Naoto Kan as president of the ruling DPJ and therefore prime minister of Japan.

Koshiishi had close ties to Ozawa[1] and Noda appointing him was an attempt at achieving party unity.

[7] The attempt was ultimately unsuccessful, as despite Koshiishi attempting to smooth over differences,[8] following Noda's administration passing an increase to the consumption tax from 5% to 10% through the lower house of the diet Ozawa and dozens of lawmakers from his faction left the party in July 2012 to form People's Life First.