He violently opposed a conspiracy that came to be known as the Maki Clan Incident, which his father and Maki no kata (his father's second wife) collaboratively formed in 1205 with the intention of deposing the third shogun, MINAMOTO no Sanetomo, and helping Tomomasa HIRAGA, their daughter's husband, become the new shogun; he then forced his father to leave the shogunate in cooperation with his elder sister Masako, and became the second regent and director of the Administrative Office.
元久2年（1205年）に父が後妻の牧の方と共謀して3代将軍・源実朝を廃して娘婿の平賀朝雅を新将軍に擁立しようとした陰謀（牧氏事件）には猛反対し、姉政子と共に父を強制的に幕府から排除して政所の別当となり、第2代執権となった。