At the Battle of Sekigahara in 1600, he accompanied Ieyasu's main force, and was assigned as Assistant Deputy General of the Eastern Camp along with Tadakatsu HONDA, becoming a key player in the command the Eastern Camp.
慶長5年（1600年）の関ヶ原の戦いでは家康本軍に随行し、本多忠勝と共に東軍の軍監に任命され、東軍指揮の中心的存在となった。