Koga is from Kokura, Fukuoka and began judo at the junior high school days.[1] In 1961, When he was a student of Nihon University, was chosen to represent Japan in the World Championship held in Paris and got a bronze medal.
Koga began working for the Yawata Steel[1] after graduation from university in 1962.
Koga also participated All-Japan Championships 8 times and won a bronze medal in 1965.
- All-Japan University Championships 1st