[9] Nishioka won the annual Japanese boxing series, West Japan Rookie King Tournament in the super bantamweight division at the Osaka Prefectural Gymnasium in September 1995.
[16] Nishioka went down a weight division to capture the Japanese bantamweight title at the Osaka Municipal Central Gymnasium on December 29, 1998, the last day when Tatsuyoshi was a world champion.
In June 2000, Nishioka challenged the WBC bantamweight champion Veeraphol Nakonluang-Promotion in Takasago, Hyōgo, and lost via a unanimous decision.
The second world title shot was watched by 12,000 spectators[21] at the Yokohama Arena in September 2001, and was a very close fight in which the scores were 115–113 for Nishioka, 116–113 for Veeraphol, and 114–114 even.
[24] In December 2002, he returned to the ring for the first time in fifteen months to gain a first-round knockout victory at the Mandalay Bay Resort and Casino in Las Vegas, Nevada.
[15] In his fourth world title shot against Veeraphol in March 2004, he lost via a unanimous decision by a wide margin of points at the Saitama Super Arena.
[14] Nishioka captured the WBC super bantamweight interim title against Thai's Napapol Kiatisakchokchai at the Pacifico Yokohama on September 15, 2008.
[38] 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.
[47] On October 24 of the same year, Nishioka fought a mandatory bout against Rendall Munroe at the Ryōgoku Kokugikan, and defeated him by a convincing unanimous decision to extend his defending streak to five matches.
[19] When Nishioka was named Japan's Fighter of the Year in 2010, he received a video message from Fernando Montiel demanding a fight at the awards ceremony in January 2011.
Nishioka said he was willing to fight anytime soon afterward[49] and his camp intended to arrange a match, but the contract was not finalized as Montiel lost to Nonito Donaire in February.
[50] On April 8, 2011, he knocked out Argentina's Mauricio Javier Muñoz in the ninth round in his sixth defense which was moved from the Ryōgoku Kokugikan in Tokyo to the World Memorial Hall in Kobe due to the Tōhoku earthquake.
[61] Nishioka lost to Nonito Donaire by technical knockout in 1:54 in the ninth round at the Home Depot Center in Carson, California on October 13, 2012.