関東学院大学
Koizumi grew up in Yokosuka, his father's home district, attending Kanto Gakuin University in Yokohama from elementary school through university with a Bachelor of Economics degree in 2004. When he was a student in junior high and senior high schools, he was engrossed in playing sports, especially baseball. He received his master's degree in political science from Columbia University. He spent one year as a part-time research fellow at the Center for Strategic and International Studies and as Young Leader of the Pacific Forum CSIS before returning to Japan in 2007. After that, he worked as a private secretary of Junichirō, his father.