On December 10, 2006, she alone out of five (originally six, but Ayami Masuda dropped out) audition finalists was chosen to join Morning Musume. An article published on December 11, 2006 revealed that Mitsui wouldn't be present in the 57th edition of Kōhaku Uta Gassen but would instead be featured in the group's 32nd single, Egao Yes Nude, which was released on February 14, 2007, following her first public appearance on the last round of the Hello! Project Winter 2007 Tours, ~Shūgetsu 10th Anniversary~ (～集結! 10th Anniversary～, ~Shūgetsu 10th Anniversary~?), which took place between January 27, 2007 and January 28, 2007 at Saitama Super Arena.