Tadamasa joined up with the eastern army of Hidetada TOKUGAWA in the Battle of Sekigahara of the same year, and participated in the attacks against Ueda-jo Castle which was the base for Masayuki SANADA of the western army.
同年の関ヶ原の戦いでは東軍に与し、徳川秀忠の軍勢に属して西軍に属した真田昌幸の居城・上田城攻撃に参加した。