In January, 2001, he challenged Freddie Norwood for the WBA featherweight title, but lost by 9th-round TKO.
Koshimoto got his second world title shot on January 29, 2006, fighting WBC featherweight champion Injin Chi.
He won by 12-round decision, becoming the oldest Japanese boxer to win a world title, at 35 years of age.
[1] Koshimoto was paid almost nothing for the fight, since his gym had funded all the money needed for the title match.
On July 30, 2006, he fought WBC Youth Champion Rodolfo López for his first defense, losing by 7th-round TKO.