In 2000, when Tsuji was twelve years old, she auditioned for the pop group Morning Musume. Group producer Tsunku originally planned on selecting only three members; Rika Ishikawa, Hitomi Yoshizawa, and Ai Kago were chosen. He made the decision to add Tsuji as a fourth member, creating Morning Musume's fourth generation. "Happy Summer Wedding" was released on May 17, 2000, marking Tsuji's debut just one month before turning age thirteen. In January 2001, she formed Mini Moni with Mari Yaguchi and Ai Kago.