The "earliest ancestor," Tadashige SUMITOMO, who served Shogun Muromachi, had a son called Yorisada who served Yoshiharu ASHIKAGA, and Yorisada's son Sadanobu called himself Osakabe-sho.
室町将軍に仕えた「始祖」・住友忠重の子・頼定は、足利義晴に仕え、頼定の子・定信は刑部承と称した。