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His music career began on February 1, 1984 with "Monica" (later remade in Cantonese sung by Leslie Cheung) and he won 8 music awards in the same year. After taking a short break in 1988, he came back as a lead singer in the band Complex with Tomoyasu Hotei (former guitarist of Boøwy) and "Be My Baby" was released. Complex remained as one of the most popular bands in Japan until 1990.
After "Complex", Kōji released series of hit singles such as "Setsunasa O Korosenai", "Kiss Ni Utarete Nemuritai" and "Boy's Life". Kōji completed a "20th Anniversary Tour" at Budokan on February 1, 2005.
In 2006 he recorded "One World," the theme for the 2006 motion picture Kamen Rider Kabuto: God Speed Love. More recently, he had teamed up with popular Japanese DJ TWINS to release a single "Juicy Jungle". Also, DJ TWINS had released an album with remixes of several of Kōji's previous hits.
With this new sound-disco revisit, Kōji redefined himself like he does year after year. Throughout all his career, it could be considered that Kōji had outgrown from an idol to an artiste that explore the music boundary. He writes his own songs, produced and arranged his own albums and also played most of the instruments in it. He had just released a remix album of his old hits-titled-DISCO K2. In April 2007, a new single "BABY JANE" was released.
- Shatterer (1988)
- The Guys from Paradise (2001)
- Kamen Rider × Kamen Rider W & Decade: Movie War 2010 (2009) - Sokichi Narumi
- Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core (2010) - Sokichi Narumi
- Sword of Desperation (2010) – Hayatonosho Obiya
- Rurouni Kenshin (2012) – Udō Jin-e
- Himitsu – Top Secret (2016) – Kiyotaka Kainuma
- Tenchijin (2009) – Oda Nobunaga
- Sunao ni Narenakute (2010) – Ryōsuke Nakajima
- Yae no Sakura (2013) – Saigō Takamori
- Downtown Rocket (2015) - Michio Zaizen
- Kuroshoin no Rokubei (2018) - Rokubei
- Magi (2019) – Oda Nobunaga
The Good-Bye, Sayuri Iwai, Yasuko Kuwata
| Shinjuku Music Festival for Gold Prize
1984 (with : Yukiko Okada)
Shigeyuki Nakamura, Minako Honda