The Secret Key to the Heart Sūtra 般若心経秘鍵 （T. 2203A）, a work in which Kūkai （774–835） interpreted the Heart Sūtra 般若心経 in his own way, is considered an important work in the Shingon sect. Since the Heian period, numerous lecture records and commentaries on the Secret Key to the Heart Sūtra have been created, and discussions about various issues related to the text have been offered by Shingon scholars. In this paper, we address the question whether the Heart Sūtra can be considered an esoteric sūtra, investigated how medieval Shingon scholars interpreted it, as a partial consideration of the formation of religious studies in the Shingon sect. We found that not all Shingon scholars positioned the Heart Sūtra in the same way. For example, Raiyu 頼瑜 （1226–1304） took it as an exoteric sūtra, while Yūkai 宥快 （1345–1416） considered it just as other esoteric sūtras.
