While Ieyasu was away from Osaka, Mitsunari gathered his army and clashed with the returning Eastern Army (TOKUGAWA side) at Mino (Gifu Prefecture), leading to the Battle of Sekigahara, where Yoshiaki attacked Gifu and Oogaki Castles in early skirmishes, and fought against Mitsunari ISHIDA's forces in the main battle.
家康らの大坂留守中に三成らが挙兵し、引き返した東軍（徳川方）と美濃（岐阜県）で衝突して関ヶ原の戦いに至ると、嘉明は前哨戦である岐阜城攻めや、大垣城攻めにおいて戦い、本戦では石田三成の軍勢と戦って武功を挙げた。