The Amakasu Incident (Amakasu jiken) was the murder of two prominent Japanese anarchists and a young boy by military police, led by Lieutenant Amakasu Masahiko, in September 1923. The victims were Ōsugi Sakae, an informal leader of the Japanese anarchist movement, together with the anarcha-feminist Itō Noe (his wife), and Ōsugi's child nephew.
During the chaos that followed the catastrophic 1923 Great Kantō earthquake, Japanese authorities killed many dissidents and ethnic Koreans in what became known as the Kantō Massacre. Itō, Ōsugi, and his nephew were arrested on 16 September. According to writer and activist Jakucho Setouchi, Itō, Ōsugi, and his 6-year-old nephew were arrested, beaten to death and thrown into an abandoned well by a squad of military police led by Lieutenant Masahiko Amakasu. According to literary scholar Patricia Morley, Itō and Ōsugi were strangled in their cells. Both accounts agree that both or all of the prisoners were brutally executed without a trial, where convictions and death sentences for the two adults would have been almost guaranteed. These killings, which became known as the Amakasu Incident, sparked widespread anger. The historian John Crump argued that "once again, the most able anarchist of his generation had been murdered," echoing the execution of Kōtoku Shūsui in the High Treason Incident just twelve years prior.
Following nationwide outcry, Amakasu was court-martialed and sentenced to 10 years in prison. When Hirohito became Emperor of Japan three years later, Amakasu was released. He studied in France and became a special agent for the Imperial Japanese Army in Manchuria. When Japan surrendered in August 1945 he killed himself with potassium cyanide.