 He was elected to the House Councillors for the first time in 1998 and re-elected in 2004 and 2010.In January 2016 he announced that he would not seek a fourth term and was replaced by Democratic Party candidate at the July 2016 election.Koshiishi became general secretary of the DPJ on August 31, 2011, the first upper house member of the party to occupy the position, and at 75 years of age, the oldest person ever to become general secretary of the party.