The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for generating a MAC CE in a carrier aggregation system, the method comprising: configuring a plurality of Timing Advance Groups (TAGs), wherein each of the plurality of TAGs corresponds to a respective TAG index; determining a size of a TAG index field in a MAC CE depending on a highest TAG index among the indices of the plurality of TAGs; and generating the Medium Access Control (MAC) Control Element (CE) according to the size of the TAG index field.