Toward the end of the Shogunate, he served at the clan's office in Kyoto, where he became acquainted with Loyalists of other clans. He advocated unification of the Shogunate and the Imperial Court.
After the Meiji Restoration (1868), he was appointed director of the Kumamoto Middle School.
When Takamori Saigo rebelled against the government (1877), he and Kichijuro Ikebe, also a former retainer of the Kumamoto Clan, organized the Kumamoto Volunteers Band to assist Saigo. The band marched to Hitoyoshi, Kumamoto Prefecture, then fought in the Hiuga District (Miyazaki Prefecture) but was defeated and surrendered at Nagai. Soshiro was taken to Nagasaki, where he was executed.