In 1986, Koyama graduated from Ritsumeikan University, where he studied law. From there, he enrolled in Toho Gakuen College, majoring in drama. Upon graduating in 1988, Koyama joined the Haiyuza Theatre Company. He played the part of a servant in “The Idiot”, his first production. It wasn’t until he was chosen to play the part of Sebastian in "Twelfth Night" that he was noticed by the public.
From plays, Koyama branched off into television and movies. Some of his roles involved the Japanese dubbed versions of American movies and television shows such as “Ocean's Eleven”, “Matrix“, and “ER”. Incidentally enough, his dubbing for George Clooney’s character on ER paved the way for him to be the pick to dub every movie character portrayed by Clooney since “Peacemaker”. He has also portrayed numerous leading characters in major plays.