He was a head of the Soryo (governing) family of the Taira clan of Boso Province and it is said that MINAMOTO no Yoritomo succeeded to raise an army owing to the support from Hirotsune, the most powerful man in the Togoku (eastern country).
房総平氏惣領家頭首であり、東国最大の勢力であった広常の加担が源頼朝挙兵の成功を決定付けたとも言われる。