As a result of his father's position, he had the chance to hear Prime Minister Hara Takashi's opposition to extending the franchise.

In late 1923, outraged by the massacre of Japanese leftists and Koreans in the aftermath of the Great Kanto earthquake, he made up his mind to carry out the assassination.

Crown Prince and Regent Hirohito was on his way to the opening of the 48th Session of the Imperial Diet when Nanba fired a small pistol at his carriage.

On November 13, 1924, he was found guilty of high treason at an extraordinary session of the Supreme Court of Japan.

[3] His father and his married sister exiled themselves to Java, Dutch East Indies in order to escape the disgrace which Nanba, by his act, had brought upon the family.