In English-speaking academic world, Martin Heidegger's ”The Question Concerning Technology” has been one of the most influential texts in philosophy of technology. There are, however, considerable limitations in traditional interpretations: they fail to thoroughly grasp the essence of technology, which is the most critical concept of the text. Through careful and close reading, I wish to show in this essay the complexity and richness of the essence of technology, and I argue that this concept cannot simply be regarded as Gestell, but should instead be considered as mystery (”Geheimnis”).