In 1600, when Ieyasu raised an army for the subjugation of Kagekatsu UESUGI, he departed for the front along with Ieyasu; in the meantime, when Mitsunari raised the western squad in Osaka, he fought in the Battle of Sekigahara as the military commander of the eastern squad.
慶長5年（1600年）に家康が会津の上杉景勝討伐の兵を起すと家康に従って出陣し、出兵中に三成らが大坂で西軍を率いて挙兵すると、東軍武将として関ヶ原の戦いにおいて戦う。