She joined Morning Musume in 2001 along with 3 others- Ai Takahashi, Asami Konno and Makoto Ogawa.

In 2001, Niigaki joined Morning Musume as the fifth generation along with Ai Takahashi, Asami Konno, and Makoto Ogawa.

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.

In the summer of that year, she also debuted in a shuffle unit as part of Happy 7, with two other members of her generation (Ai Takahashi and Makoto Ogawa).

[2] Niigaki, along with many other members of Morning Musume at that time, starred in a drama entitled Angel Hearts which was released in 2002.

In October 2007, Niigaki was placed in the unit Athena & Robikerottsu along with fellow Morning Musume member Aika Mitsui, as well as Saki Nakajima and Chisato Okai of Cute.

It then was announced in July 2008 that Niigaki and Ai Takahashi would play the 70s idol J-pop duo Pink Lady in the TV drama Hitmaker Aku Yū Monogatari.

[8] In an interview Niigaki stated that "The movies that Disney makes are full of dreams, but when I have children, I'd like them to know about war, the pain of death and such things, so I hope they will watch [anime].

Niigaki and Takahashi became the longest-serving members in Morning Musume history, on January 17, 2009, breaking the four-year-old record set by Kaori Iida.

Niigaki was later assigned to be a part of the new shuffle group ZYX-α with Koharu Kusumi, Maasa Sudo, Momoko Tsugunaga, Erika Umeda, Chinami Tokunaga, Saki Ogawa, and Ayaka Wada.