He became the eldest son because of the death of Munetoki HOJO, his elder brother, in the Battle of Ishibashiyama, for which Yoritomo took up arms, and was entitled to succeed his father as the head of the Hojo clan
頼朝の挙兵である石橋山の戦いで兄北条宗時が戦死した為、嫡男となり、北条氏を継承する権利を与えられた