Kobayashi made his debut in April 1992 in the bantamweight division, losing by 4th-round decision.
Kobayashi made his first defense in September 2001, winning by split decision.
He fought Alexander Muñoz (undefeated in 21 fights, all won by KO) in March 2002, for his second defense, and lost by TKO in the 8th round to lose his title.
After his loss to Munoz, Kobayashi declined an offer to challenge Masamori Tokuyama for the WBC Super flyweight title,[verification needed] and retired from boxing.
He frequently appears as a commentator in Japan for boxing title matches.