文化放送
As a broadcasting student at the Nihon University College of Art, he began working under Yasuji Hayashi of Nippon Cultural Broadcasting. Hayashi, who found him an "interesting character", asked Koyama to begin writing scripts for broadcast. In Koyama's third year he branched out into television screenwriting, making his debut with the late-night show "11pm". Continuing this screenwriting after graduation, Koyama began getting attention for his work on Fuji Television's "Kanossa no Kutsujoku" (1990–1991), a late-night show which "took various modern-day social phenomena and products and explained them satirically by presenting them as historical events and folklore".