[1] After graduating from Tenri University, Ninomiya entered the Shoki Juku under the instruction of Isao Okano.

[2] He joined the Fukuoka Prefecture police force in April 1972,[2] and placed third in the All-Japan Judo Championships that year.

He won a gold medal in the heavyweight division of the Asian Judo Championships in 1970 and the open weight division of the 1973 World Championships held in Lausanne, Switzerland by defeating future Olympic gold medalist Haruki Uemura.

However, the Japanese Olympic team had already decided on Haruki Uemura and Sumio Endo as the representatives for the Openweight 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.

[citation needed] Ninomiya retired after competing in the 1978 Jigoro Kano Cup along with Isamu Sonoda.