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Nishioka was born in Kakogawa, Hyōgo, Japan, in July 1976. He started boxing in the fifth grade of elementary school on the recommendation of his father who rans a tavern. He compiled an amateur record of 10–2, and turned professional.

Nishioka captured the WBC super bantamweight interim title against Thai's Napapol Kiatisakchokchai at the Pacifico Yokohama on September 15, 2008. Just after the fight, Veeraphol whom he fought four times, and who served as Napapol's special coach, climbed into the ring to congratulate Nishioka. Nishioka inherited the fullversion of the WBC super bantamweight title on December 18 of that year, when its previous holder, Israel Vázquez, was stripped for medical reasons. He defended that title by knocking out Mexico's Genaro Garcia in the final round, again at the Pacifico Yokohama on January 3, 2009.

On May 23, 2009, Nishioka successfully defended his title against Jhonny González at the Monterrey Arena, Mexico. Nishioka recovered from a knockdown on the first round and went to knock out González in the third round.[35] Nishioka's left blows including his left crosses have often been called Monster Left from those days.[2][3] He succeeded in the third defense via a third round technical knockout against Ivan Hernández at the Yoyogi National Gymnasium on October 10 of the same year.