In 1891, over the Ashio Copper Mine Mineral Pollution Incident, he received questions from Shozo TANAKA in the Imperial Diet but responded that he did not understand the purport of the questions (Mutsu's second son, Junkichi, had been adopted by Ichibei FURUKAWA, the manager of Ashio Copper Mine).
明治24年（1891年）に足尾銅山鉱毒事件をめぐり、帝国議会で田中正造から質問主意書を受けるが、質問の趣旨がわからないと回答を出す（二男潤吉は足尾銅山の経営者、古河市兵衛の養子であった）。