In the philosopy of Aristotle, it is said that the substance is the most important notion. But Aristotle himself didn't make clear the metaphysical distinction between the essential substance i. e. the essence and the existential substance i. e. the existence. Thomas Aquinas also had the deep insight into the problem of the substance. He had clearly distinguished the important difference between the essence and the existence by using the word of "actus" and "potentia" which are the basic notions of the metaphysics. In "De ente et essentia", as the title shows, "esse" isn't entitled directly. But by taking into consideration on the notion of "ens" and "essentia", Thomas Aquinas had been confirming that "esse" was more basic notion for the metaphysics than "essentia". And he had finally arrived at the conclusion that "esse" (ειναι) i. e. the existence held priority to "essentia" (το τι ην ειναι, τι εστιν) i. e. the essence. I have tried in this paper to consider the basic notion of "ens", "essentia" and "esse", and to show how "esse" underlies "essentia" originally by reading the text "De ente et essentia" (and "Metaphysics").