A Tosa samurai, he was studying medicine in Nagasaki when he met Sakamoto Ryoma and joined the Kaientai, becoming its secretary. Nagaoka had a helping hand in drafting the Eight Point Plan. Nagaoka was also known to be very anti-Christian.
- Hillsborough, Romulus. RYOMA- Life of a Renaissance Samurai. Ridgeback Press, 1999
- Jansen, Marius B. Sakamoto Ryoma and the Meiji Restoration. Columbia University Press, 1994.