Tetsunosuke Seki was born in 1824, Ibaraki Prefecture.
A retainer of the Mito Clan (Ibaraki Prefecture), he was in charge of local administration. He changed his name to Kannosuke Miyoshi and toured the Hokuriku and Chugoku districts, calling on the masses to rise for the cause of the Meiji Restoration (1868).
He participated in the assaassination of Naosuke Ii, the Shogunate Prime Minister at Sakuradamon Gate of Edo Castle by a band of Mito samurai (1860). Was arrested in 1861 and executed.