However, he was knocked out by Jirō Watanabe in the first round of the final match of All-Japan Rookie King Tournament at the Osaka Prefectural Gymnasium in February 1980.

He then had a fifteen-fight winning streak including ten knockouts over the former world ranked boxer Facomron Vibonchai, the national champions of Thailand, Philippines, Korea and others.

[1] On January 18, 1984, Kobayashi fought against Frank Cedeño for the WBC and lineal flyweight titles at the Korakuen Hall.

[2] For that fight, Isamu Mitsuhira, the former trainer of Kyokutō Boxing Club to which Yoshiaki Numata belonged, served as his special coach.

[5][6] In his first defense in April 1984, he was knocked out by the mandatory challenger[4] Gabriel Bernal in the second round at the Korakuen Hall.