Masanari and Yorikatsu HIRAOKA had secret communications with Ieyasu TOKUGAWA during the battle of Sekigahara in 1600, and Masanari was able to convince Hideaki to come over to the Eastern army.
慶長5年（1600年）の関ヶ原の戦いでは、平岡頼勝と共に徳川家康と内通し、秀秋を東軍に寝返らせることに成功した。