Because his elder brother died young, he became head of the family and was appointed General of the Imperial Guard and Lord of Omi. After he became a Buddhist priest, he called himself Gukan.
When Emperor Godaigo returned from his exile on Oki Island (1331), he pledged loyalty to the Emperor together with Taketoki Kikuchi and Sadatsune Shoni. But when Taketoki attacked Hidetoki Hojo, Sadamune switched to Hidetoki's side and killed Taketoki. When Emperor Godaigo's forces were victorious at Rokuhara (Kyoto), he killed Hidetoki for fear that the Emperor might punish him.
When Takauji Ashikaga attacked Kyoto (1336), he asked for Sadamune's help. Sadamune, together with Hiroyo Ouchi, fought on Takauji's side. After Takauji was defeated, Sadamune advised him to flee to Kyushu. When Takauji came back from Kyushu and recovered Kyoto, Sadamune fought in many battles for Takauji.