The gravitational theory has one natural parameter, the dimension D of the spacetime. In the limit of infinite D the gravitational theory is simplified and its structure becomes trivial. In this seminar, using this remarkable property of the gravity at large D, we introduce an approximation method in gravitational theory. As examples we solve the scalar wave propagation and black brane instability. These examples show the usefulness of the large D approximation. Furthermore we discuss the universality of the near horizon metric in the large D limit. This fact instructs us that there may be a certain effective theory of large D gravity.