In the theory of Luhmann, two terms of "observation" (Beobachtung) and "observer" (Beobachter) are used in a peculiar sense and play an important part: "Observation" means all of our performances and an "observer" means an actor. In this paper, the notion of "observation" will be examined, with my attention to the Luhmann's theory, especially to his significant terms: "Selbst-Referenz", "Enttautologisierung" and "Ereignis". So its theoretical and factual connotation in the modern social theories will be explained. First, what the viewpoint it is to take an observer for an actor will be recognized in the context of understanding sociology. The importance of the notion of "obseavation" will thus come clear in terms of men's relation to world. Secondly, what a great change this viewpoint has brought to the traditional idea of the "world" will be revealed, by making reference to the calculus of G. Spencer-Brown. Finally, as a moment to give all the beings in the world and the "world" itself a possibility that they will acquire their self-referential structures, the existence of "the other" will be pointed out.