In the Battle of Sekigahara in 1600, he joined the army of Mitsunari ISHIDA, the Western Army, and kept to Sawayama-jo Castle, the headquarters of Mitsunari, but when the main forces of the Western Army including those of Mitsunari were devastated in the main campaign on October 21 and the troops of Hideaki KOBAYAKAWA and other lords who belonged to the Western Army but betrayed them in favor of the Eastern Army, closed to the castle, Moritomo switched sides to Hideaki, betrayed Mitsunari and forced his family to suicide.
慶長5年（1600年）の関ヶ原の戦いでは西軍の石田三成に属して、三成の居城である佐和山城に籠もっていたが、9月15日の本戦で三成ら本隊が壊滅し、小早川秀秋ら東軍の寝返り部隊が攻め寄せてくると、守知は秀秋と内通して裏切り、三成一族を自刃に追い込んだ。