Koga was born in Kitashigeyasu, Saga, Japan. He began judo in elementary school. He traveled to Tokyo during junior-high school to enter Kodogakusha, a judo school later attended by Olympic gold medalists Hidehiko Yoshida and Makoto Takimoto. He continued his education at the Nippon Sport Science University, and captured several awards including five consecutive championships at the Kodokan Cup and six consecutive championships at the All-Japan Judo Championships (all in the 71 kg division). He placed 3rd in the 1987 World Judo Championships held in Essen, and was chosen to participate in the 1988 Summer Olympics, where he lost in the 3rd round of the competition. Koga returned to the Olympics in 1992 after winning the 1989 and 1991 World Judo Championships, but seriously injured his left knee during a randori (sparring) session against Hidehiko Yoshida, which prevented him from using his best technique; the Ippon Seoinage. He won the gold medal overcoming this injury, and was given the JOC Sports Award by the Japanese Olympic Committee.