帝拳プロモーション
In June 2000, Nishioka challenged the WBC bantamweight champion Veeraphol Nakonluang-Promotion in Takasago, Hyōgo, and lost via a unanimous decision. He had belonged to the JM Kakogawa Gym until that fight, and has been managed by the Teiken Boxing Gym in Tokyo under Akihiko Honda's supervision and Yūichi Kasai's guidance since September 2000.