The Ise theory, based on a letter Soun sent to Sadamoto OGASAWARA, Governor of Shinano Province, stating 'I belong to the same family as the Seki Clan of Ise,' that appears in the "Hojoki" and the "Soshu Heiranki," was advocated in 1901 by the scholar, Tsugihei FUJII, and supported by the historian, Yoshinari TANAKA.
伊勢説は『北条記』『相州兵乱記』に書かれており、早雲が信濃国守護小笠原定基に宛てた書状で「伊勢の関氏と同族である」と書いていたことを根拠に1901年に藤井継平が主張し、田中義成がこれを支持した。