Tsuboshima graduated from the Waseda University Faculty of Commerce in 1952. While attending the university, he watched all kinds of movies, both Japanese and Western, and aspired to become a film director. He joined Toho when he graduated from university in 1956. However, at that time, Toho had stopped officially hiring assistant directors due to the Toho Dispute and so he was assigned as a (a person in charge of paying dividends to actors). After three years of being a talent agent, he became an assistant director in 1955 and studied under , Yasuki Chiba, and others.