Kasai was born in Yokohama, Kanagawa Prefecture, Japan, and trained in Judo as a child before he began boxing at the age of a high school student.

Dropping out of the Senshu University in one year with an amateur record of 44–4 (24 KOs),[1] Kasai made his professional debut as a boxer managed by the Teiken Boxing Gym at the Korakuen Hall on August 12, 1989.

[2] After winning the sixth edition of the annual Japanese boxing series, the Class A Tournament in the junior featherweight division in November 1991, he experienced three away matches in the first half of 1992.

Then he learned under the instruction of Miguel Diaz at the Top Rank Gym while staying in Las Vegas, Nevada, and won two fights there at the Thomas & Mack Center and the Mirage.

In March 1994 Kasai fought against the WBA world junior featherweight champion Wilfredo Vázquez as an undefeated challenger at the Tokyo Metropolitan Gymnasium where he was knocked out at two minutes five seconds of the first round.