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.
After "Complex", Kōji released series of hit singles such as "Setsunasa O Korosenai", "Kiss Ni Utarete Nemuritai" and "Boy's Life".
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".
In 2011, Kōji and Tomoyasu Hotei reunited as Complex for two concerts at the Tokyo Dome to benefit victims of the Tohoku earthquake and tsunami.