In the Kokoku era, after taking the side of the Southern Court (Japan) (headed by Emperor Godaigo), Yukimune placed his military base in the Isa Castle of Isa County (Hitachi Province), a main castle of his relative Ise clan, and fought against KO no Morofuyu of the Northern Court (Japan) (headed by the Ashikaga clan).
興国年間には南朝_(日本)方（後醍醐天皇方）として同族の伊佐氏(常陸国)とともに常陸国伊佐郡(常陸国)の伊佐城に拠り、北朝_(日本)方（足利方）の高師冬等と戦う。