His first world title shot against WBA featherweight champion Alexis Argüello ended in a fifth round knockout loss,[7] in front of 16,000 spectators at the Kuramae Kokugikan in Tokyo in October 1975.
On October 9, 1976, Kobayashi moved down a weight class and dethroned Rigoberto Riasco as the WBC and lineal junior featherweight champion while being watched by 9,000 spectators at the Kuramae Kokugikan.
[11][12] However, he lost the title in his first defense against Dong-Kyun Yum via a majority decision at the Jangchung Gymnasium in Seoul, South Korea, on November 24 of that year.
[13][14] In January 1978, Kobayashi challenged Wilfredo Gómez to regain the WBC junior featherweight title in front of 10,000 spectators[14] at the Kitakyūshū Municipal Gymnasium in Fukuoka, but was knocked out in the third round.
His manager Takahashi later told that he realized the importance of short punches when Kobayashi lost to Pedroza and when he brought up Leopard Tamakuma to be a world champion he taught it to him thoroughly.