Dōgen (道元) approaches for Buddhist practice as “Shūshō-Ittō (修証一等)” which nondually integrates practice with nirvāṇa. If practice and nirvāṇa are equal, we can understand the theory of ascetic practice in Dōgen as a theory of attaining Buddhahood. Previously, scholars have questioned whether there is some relation with “Tendai Hongaku shisō (天台本覚思想),” there is no agreed upon stance. As Hanano Jūdō suggests, the reason is that the Tendai Hongaku theory is not specified in Dōgen’s writings. Comparing the theory of ascetic practice of Dōgen with Japanese Tendai, we see an aspect of the ideas of Annen (安然) and Shōshin (証真). This is important, and leads to elucidating the formation of “Dōgen-Zen (道元禅)” in thinking about Tendai Hongaku shisō. This approach may lead to a fresh view of Dōgen’s ideas of practice.