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.

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