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.

[4] He won the gold medal overcoming this injury, and was given the JOC Sports Award by the Japanese Olympic Committee.

Koga announced his final retirement from competition in 2000, and became the head coach for the All-Japan women's judo team.

One of his pupils, Ayumi Tanimoto, won the gold medal in the -63 kg division at the 2004 Summer Olympics.