In 1863, he petitioned the Domain through administration Masanosuke SUFU to send him abroad and smuggled himself into England with Hirobumi ITO, Yozo YAMAO and Masaru INOUE as one of five outstanding persons of Choshu, which made him convert to the opinion of the opening the country to the world, realizing the difference in the national power, and then returned to Japan immediately together with Ito to try to make an effort for peace negotiations when the Shimonoseki War began.
文久3年（1863年）、執政周布政之助を通じて洋行を藩に嘆願、伊藤博文・山尾庸三・井上勝らとともに長州五傑の一人としてイギリスへ密航するが、留学中に国力の違いを目の当たりにして開国論に転じ、下関戦争では伊藤とともに急遽帰国して和平交渉に尽力した。