This article aims to analyze and elucidate Nietzsche’s concept of decadence, which has often been mentioned in Nietzsche studies and thus feels familiar, but in fact has not been thoroughly analyzed. As Nietzsche describes many phenomena in terms of decadence and the extension of the term may be seen as too broad, some would think we cannot construct one picture of decadence from Nietzsche’s various descriptions and consider that it is sufficient to say it has a general meaning of decline or decay. However, this article seeks to combine Nietzsche’s scattered remarks on decadence together into a coherent picture. I argue that the essence or the fundamental principle of decadence is the lack of self in the sense of the loss of the fundamental instinct as the center within the person. Grasping this principle, we can understand how the various phenomena Nietzsche describes as decadence belong together; furthermore, we can understand why Nietzsche sees Christians or Socrates, despite their struggle for improvement, as decadents.