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Hiroyasu Koga (古賀浩靖 Koga Hiroyasu) is a former Tatenokai (楯の会) member and kaishakunin responsible for the decapitations of Yukio Mishima and Masakatsu Morita during their seppuku on November 25, 1970. He studied law at Kanagawa University, and intended to become a lawyer.
Koga, known by the nickname Furu-Koga to distinguish him from another Tatenokai member with the name Koga (小賀), was a skilled practitioner of kendo (swordsmanship). It was originally planned that Mishima would be decapitated by Masakatsu Morita, the Tatenokai's student leader; however, Morita was not trained in the sword and failed, at which point Koga stepped in to complete it. Koga then decapitated Morita as part of Morita's own seppuku.
After the trial, he and two other members were sentenced to four years' penal servitude. He married the daughter of Seicho Taniguchi, the leader of Seicho no Ie, and changed his surname to Arechi (荒地). As of 2005, he was a practicing Shinto priest at a shrine on Shikoku.
- Koga, Masayoshi; Koga, Hiroyasu; Ogawa, Masahiro; Date, Munekatsu (1972). 裁判記錄三島由紀夫事件 (Saiban kiroku Mishima Yukio jiken) [Court Transcript of the Yukio Mishima Incident] (in Japanese).