Abstract: Basic norm is one of core cotegaries in Kelsen's legal theory, which is very important to his pure theory of law. But Kelsen didn't make a clear definition of the nature of the basic norm. At first, Kelsen modified the basic norm as transcendental-logical presupposition according to the neo-Kantianism, and then he argued that the basic norm becomes a genuine fiction in the sense of Vaihinger's philosophy of “as-if”. Why? Because Kelsen's purpose that is to make a neo-Kantian foundation for the legal system didn't succeed. His purpose failed because Kelsen didn't stick to the neo-Kantianism thoroughly, when he made the transcendental argumentation of the basic norm.