Career
After graduating from Tenri University, Ninomiya entered the Shoki Juku under the instruction of Isao Okano. He joined the Fukuoka Prefecture police force in April, 1972, and placed third in the All-Japan Judo Championships that year. However, the Japanese Olympic team had already decided on Haruki Uemura and Sumio Endo as the representatives for the open weight and heavyweight divisions, and Ninomiya was forced to enter the competition as a half heavyweight (-93 kg), shedding over 7 kg from his usual competitive weight.
Ninomiya retired after competing in the 1978 Jigoro Kano Cup along with Isamu Sonoda.
He and Sonoda were rivals and friends for over 30 years, having been born on the same year, entered the same police force, competed in the same World Championships and Olympics, and having retired at the same time. After serving as an advisor for several local and prefectural level judo committees, he became a judo instructor at the Nishinippon Institute of Technology in 2007.