Disclosed in the present application is a method by which a terminal reports a beam index to a base station in a wireless access system. The beam index reporting method comprises the steps of: receiving a plurality of reference signals from the base station; measuring a plurality of beams by using the plurality of reference signals; reporting an index of a first preferential beam among the plurality of beams to the base station; and reporting, to the base station, information on at least one second preferential beam determined on the basis of the first preferential beam, wherein the information on at least one second preferential beam is information on an index difference between the first preferential beam and the at least one second preferential beam.