He played in Nippon Professional Baseball and in the American minor leagues, while later becoming a minor league manager in the United States and a front office worker and coach in Japan.
Prior to playing professionally, Koga attended Kumamoto Kogyo High School and then Kinki University.
In 1966, he pitched for the Decatur Commodores in the San Francisco Giants organization, going 7-8 with a 3.20 ERA in 31 games (15 starts).
In 1968, with the Lodi Crushers in the Chicago Cubs system, he went 1-4 with a 4.25 ERA in 26 games (five starts).
From 1990 to 1992, he managed the California League's Salinas Spurs, finishing with losing records each season.