Before debuting with Tokio, Kokubun, like other Tokio members, was a backup-dancer for Johnny's Entertainment boy bands, including Hikaru Genji.[3] In 1990 he became the keyboardist of Tokio, but it was not until 1994 that the band debuted.[3]
In 2004, Kokubun took a break from Tokio to form a temporary group with KinKi Kids member Tsuyoshi Domoto.[3] Together they released the theme song to Fantastipo, a movie in which they starred.
As an actor
Kokubun began his acting career with a role in Abunai Shōnen III, a 1989 Japanese drama. Since 1993, he has played parts in drama shows and movies such as Fantistipo and Shaberedomo Shaberedomo.
He has hosted Gachinko! with other Tokio members.[2] Currently Kokubun hosts Kaitai Shin Show and Shonen Club Premium on NHK, Tetsuwan Dash and Guruguru Ninety-Nine on NTV, R30 and Sekai Kurabete Mitara on TBS, Mentore G on Fuji TV, and Aura no Izumi on TV Asahi.[4]