The Okamoto Daihachi incident (岡本大八事件) of 1612 refers to the exposure of the intrigues involving the Japanese Christian daimyō and retainers of the early Tokugawa shogunate in Japan.
Arima Harunobu, the daimyo of Hinoe Domain, became one of the most important supporters of the Church in post-Sekigahara Japan as Tokugawa Ieyasu became shōgun and purged his enemies, like the influential Christian lord Konishi Yukinaga.
[2] The same captain-major came to Nagasaki on the Nossa Senhora da Graça to trade in 1609, and Arima Harunobu took this opportunity to seek permission from Tokugawa Ieyasu to avenge his dead men.
[3] Harunobu apparently felt that his efforts during the Nossa Senhora da Graça incident warranted further rewards — namely the return of the territories Isahaya and Kōjiro (神代) in Hizen Province that the Arima lost during the Sengoku period.
[4] To this end, Harunobu bribed Okamoto Daihachi (岡本大八, baptismal name Paulo), a Christian aide to the rōjū Honda Masazumi, who advised Ieyasu on affairs concerning the redistribution of fiefs.
[5] Harunobu, after seeking further assistance from a Jesuit priest on this matter to no avail,[6] brought the issue directly to Ieyasu in 1612, bringing Okamoto's scam to light.
As the important port city of Nagasaki was governed directly by a shogunate-appointed official (rather than through a daimyō), this conspiracy was seen as an affront to Tokugawa rule.
The retired shōgun was further incensed to hear that Christian followers had gathered at Okamoto's execution to offer prayers and sing hymns, cementing in his mind that the creed encouraged insubordination against the shogunate.