Koga was born in Okawa on September 8, 1978. After graduating from high school, he joined the Nagoya Grampus Eight in 1997. He became a regular player as a center back from 1998. The club won the 1999 Emperor's Cup. He moved to Kashiwa Reysol in 2007. Although he played as a regular player, he lost his place due to an injury in 2010. He then moved to Júbilo Iwata in August 2010. After playing for two seasons, he moved to his local club Avispa Fukuoka in 2012. He retired at the end of the 2015 season.
