Her career began in 2002 when she successfully passed the audition for Hello! Project Kids, an all-female teen pop group within Hello! Project. Since then, she has continued to sing in that group and has become a part of two smaller groups composed of Hello! Project Kids members, Berryz Kobo and ZYX.
In 2002, Shimizu successfully passed the Hello! Project Kids Audition. Currently, she is the captain of Berryz Kobo.
Being a member of Berryz Kobo, she also participates in the band's weekly radio show Berryz Kobo Kiritsu! Rei! Chakuseki!.
In that same year, she was cast as one of the main characters in "Koinu Dan no Monogatari". Many members of Hello! Project Kids and Morning Musume participated in this film.
In 2003, Shimizu was chosen to join the group ZYX, which consisted of five Hello! Project Kids and Mari Yaguchi, who acted as a mentor. They released two singles: Iku ZYX! Fly High and Shiroi Tokyo. They remained active until 2004, when Saki and Momoko Tsugunaga were selected to become members of Berryz Kobo.
In 2004, all of Hello! Project participated in a large shuffle group that produced one single, "All For One & One For All!" This song is considered one of the themes of Hello! Project.
On December 31, 2006, Shimizu took the stage at the 57th edition of Kōhaku Uta Gassen as a backup dancer in Morning Musume's performance of Aruiteru, along with the remaining members of Berryz Kobo, Country Musume and C-ute.
In 2008 Shimizu became a member of Hello! Project's new unit High-King This shuffle unit came out with the single, "C\C (Cinderella Complex) , to promote Morning Musume's Cinderella Musical.
In 2009, Tsunku, the producer of Hello! Project, went to South Korea to announce that he intended to hold auditions. A few from Hello! Project accompanied him there, including Shimizu, Ai Takahashi, Risa Niigaki, Reina Tanaka, Linlin, and Maimi Yajima.