大政翼贊會
In 1942, Koizumi became mayor of Yokosuka. In 1937, he joined the Imperial Rule Assistance Association and was appointed deputy chairman. In 1944, he was an advisor to Prime Minister Kuniaki Koiso. He was selected to be a member of the House of Peers, but in 1946, under the occupation of Japan, he was purged from public office. He died in 1951.