In July 1863, during the Shimonoseki campaign, which began with the bombardment of the Chōshū Domain by Western forces (British Empire, United States, France and The Netherlands), Gengo headed for Shimonoseki with the messengers dispatched from the Shogunate to the Choshu while serving as the captain of the "Chōyō Maru". According to a letter to his family, the messenger's mission was to dismiss the Choshu samurais because they occupied the Ogura Clan's territory, and the messenger headed for the Ogura clan territory succeeded this. The messenger who landed in Shimonoseki was killed by foreign soldiers. Gengo was forced by the Choshu Clan to borrowed his ship for the campaign, but he refused. In 1864, he was guarded according to the general's return to the sea route. In the First Chōshū expedition, he set sailed to Aki and Buzen. He became the captain of the "Kishō Maru" and sailed to Osaka in 1866. 