Koga is from Kokura, Fukuoka and began judo at the junior high school days. 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 also participated All-Japan Championships 8 times and won a bronze medal in 1965.
- 1959 - All-Japan University Championships 2nd
- 1960 - All-Japan University Championships 3rd
- 1961 - World Championships (Openweight only) 3rd
- - All-Japan University Championships 1st
- 1965 - All-Japan Championships (Openweight only) 3rd
- "近代柔道 1997年12月号 名選手の技と技術(第50回) 支え釣り込み足&内股 -古賀武-". Baseball Magazine Sha Co., Ltd. 1997-12-20.
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