He graduated from Tokyo Christian University in 1979 and earned a master's degree in International Area Studies from the graduate school of University of Tsukuba in 1983. He studied at the International Division in Yonsei University from 1977 to 1978. He worked at the Embassy of Japan in South Korea as a special researcher for the Ministry of Foreign Affairs from 1982 to 1984. He was an Editor in Chief of the Gendai Korea (Today's Korea) journal from 1990 to 2002.[1][2]
In 2016, he started working as a guest professor for Reitaku University.
Publications
Nikkan Gokai No Shin’en (日韓誤解の深淵, The Abyss of Japan-South Korean Misunderstanding), Akishobo, (1992) ISBN4750592129
Koria Tabū O Toku (コリア・タブーを解く, Solving the Korea Taboo), Akishobo, (1997) ISBN4750597031
Yami Ni Idomu: Rachi, Kiga, Ianfu, Han-Nichi O Dō Haaku Suru Ka (闇に挑む!―拉致、飢餓、慰安婦、反日をどう把握するか, Deciphering the Darkness: Abductions, Starvation, Comfort Women and the Anti-Japan Movement), Tokuma Shoten, (1998) ISBN4198909709
Kin Sei-nichi ga shikaketa "Tainichi daiboryaku" Rachi no Shinjitu (金正日が仕掛けた「対日大謀略」拉致の真実), Tokuma Shoten, (2002) ISBN4198615977
KinSei-nichi to Kin Dai-chu (金正日と金大中――南北融和に騙されるな!), PHP Institute, (2000) ISBN4569612687
Tero kokka, Kita-Chosen ni damasareruna (テロ国家・北朝鮮に騙されるな, PHP Institute, (2002) ISBN4569624413
Kita-Chosen ni torikomareru Kankoku (北朝鮮に取り込まれる韓国――いま"隣国"で何が起こっているか), PHP Institute, (2004) ISBN4569634680
Nikkan “Rekishi Mondai” no Shinjitsu (日韓「歴史問題」の真実――「朝鮮人強制連行」「慰安婦問題」を捏造したのは誰か. The Truth About the Japan-South Korea History Issue), PHP Institute, (2005) ISBN4569643167
Kita-Chosen no "Kaku", "Rachi" ha kaiketsudekiru (北朝鮮の「核」「拉致」は解決できる), PHP Institute, (2006) ISBN4569656315
He was accused of defamation on January 9, 2015, by a former Asahi Shimbun reporter Takashi Uemura who wrote a series of articles on the comfort women issue, the first of which in 1991.[3] Nishioka accused Uemura of "fabricating" his stories on comfort women. Takashi Uemura demanded that Bungeishunju Ltd. and Tsutomu Nishioka pay ¥27.5 million in damages and issue apologies. The Tokyo District Court in 2019 rejected his claims for damages, the ruling determined that Nishioka's claims about two articles written by Uemura “did not deviate from the scope of regular reviews.”.[4]
Uemura appealed to the Supreme Court in Tokyo, which issued its ruling on the case on March 11, 2021, rejecting Uemura's appeal. The court concluded that: “An important part of the articles and claims [of fabrication] moved by Mr. Nishioka were based on the truth.”[5]