At the time of the Seinan War, Sumiyoshi went to Kagoshima immediately after the attack of Shigakuko-to (a party made up of students of military academy founded by Takamori SAIGO) on an arsenal, and he had meetings with Tsunayoshi OYAMA, a prefectural governor, to prevent further conflict, but he ended up failed partially due to a sabotage by the Shigakuko-to and returned to Tokyo.
西南戦争にあたっては、私学校党の火薬庫襲撃直後、鹿児島に入り、県令大山綱良と会談、制止に努めたが、私学校党幹部による妨害もあって不首尾に終わり帰京する。