ranking as the weekly #1 single on the Oricon charts for three consecutive weeks[24] and selling a reported total of 1,646,630 copies.

In 2000, "Koi no Dance Site" was released and it hit #2, selling over a million copies—400,000 copies away from reaching the success of "LOVE Machine". Ishiguro Aya left the group before the single was released.

In January 2000, a fourth generation audition began. The winners were Ishikawa Rika, Yoshizawa Hitomi, Tsuji Nozomi, and Kago Ai. After their debut single, "Happy Summer Wedding", Ichii Sayaka graduated to pursue her own music career.

At the end of January, the best selling Hello! Project album to date, at 2,259,510 copies, was released: Best! Morning Musume 1.

At the end of the year, four new members joined the group as the 5th Generationthrough the LOVE Audition 21: Takahashi Ai, Konno Asami, Ogawa Makoto, and Niigaki Risa, bringing the roster to 13 girls. Their first single "Mr.Moonlight ~Ai no Big Band~" sold over half a million copies.

In February, 2002, "Souda! We're ALIVE", another hit, was released.

Goto Maki's graduated late in the year, her last single with the group was "Do it! Now".

In 2003, the unique child-like tune, "Morning Musume no Hyokkori Hyoutanjima", was released in February as the group's seventeenth single. April brought the notably successful single "AS FOR ONE DAY", a synthesizer-driven song about lost love, which sold 129,893 copies and hit #1 in the Oricon charts. This was Yasuda Kei's last single.

Before the auditions for the sixth generation were held, the group released the album No.5.

Four new girls were