立憲民政党
Koizumi was born in Higashi-Kaseda, Kagoshima Prefecture (now part of Minami-Satsuma); his family were fishermen. He attended high school at night while working in a department store, and then attended law classes at Nihon University while working as a secretary to a Diet member. He graduated in 1930 and joined the Rikken Minseitō political party. He was elected to the Diet in 1937.