[1][2] He plays keyboard for Tokio, a musical group of Starto Entertainment formerly known as Johnny & Associates.

[3] In 2004, Kokubun took a break from Tokio to form a temporary group with KinKi Kids member Tsuyoshi Domoto.

Kokubun began his acting career with a role in Abunai Shōnen III, a 1989 Japanese drama.

[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] In April 2009, Kokubun became a sportscaster for the Fuji TV show Sports!.