In 2001, Niigaki joined Morning Musume as the fifth generation along with Takahashi Ai, Konno Asami, and Ogawa Makoto. Morning Musume producer Tsunku said he chose Niigaki because she "shined during the final studio recording" for the audition song that all the finalists had to sing. She debuted on the group's 13th single, "Mr. Moonlight ~Ai no Big Band~" alongside her fellow generation members. Her first appearance on a full-length Morning Musume recording was on the album, 4th Ikimasshoi!. In the summer of that year, she also debuted in a shuffle unit, as part of Happy 7, coincidentally with two other members of her generation (Takahashi and Ogawa).