Koretada became Udaijin in 970; his uncle Saneyori, who held the titles of Sessho Daijo-daijin and Uji choja (chieftain of the family), passed away in the same year, and Koretada, who was a maternal uncle to the emperor, became Sessho and Uji choja (the head of the clan).
天禄元年（970年）右大臣に拝せられ、同年、摂政太政大臣だった伯父の実頼が死去し、天皇の外戚叔父の伊尹は摂政・氏長者となる。