Kobayashi turned pro in 1962 and won the
lineal,
WBC, and
WBA super-featherweight title in 1967 by defeating
Yoshiaki Numata by 12th-round KO in a bout where Numata was down once in the 6th and three times in the 12th round. He defended his undisputed status two times before he was stripped of his WBC title on 20 January 1969 after failing to go through with an agreed rematch with
Rene Barrientos (their first fight ended in a contentious draw). Kobayashi defended his remaining titles four times before losing them to
Alfredo Marcano in 1971. He retired later that year after losing by knockout in the seventh round against
Roberto Durán in Panama. ==Professional boxing record==