Among his activities the thing that was especially conspicuous was that he helped Yoshitomo with his strong support when Yoshitomo tried to establish hegemony over the Sagami Province; it is said that Yoshiaki supported MINAMOTO no Yoshihira, the son of Yoshitomo, logistically when Yoshihira fought with his uncle MINAMOTO no Yoshikata in the Battle of Okura in 1155 (according to a theory, the mother of Yoshihira was the daughter of Yoshiaki).
殊に相模国における義朝の覇権確立の有力な後ろ盾となり、久寿2年（1155年）に義朝の子・源義平が叔父の源義賢と戦った大蔵合戦では、後方からこれを支援したとされる（一説によると義平の母は義明の娘とも言われる）。