The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for counting a DRX (Discontinuous Reception) timer in a carrier aggregation system, the method comprising: configuring a plurality of cells including at least one FDD (Frequency Division Duplex) serving cell and at least one TDD (Time Division Duplex) serving cell; and counting a DRX timer in a subframe, wherein the subframe is an uplink subframe for all TDD serving cells.