The present invention relates to a wireless communication system. The present invention relates to a method for transmitting ACK/NACK in a wireless communication system in which carrier aggregation is set, and an apparatus therefor. Specifically, the present invention relates to an ACK/NACK transmission method and an apparatus therefor, the method comprising the steps of: receiving information on a plurality of physical uplink control channel (PUCCH) resources via upper layer signaling; receiving a transmit power control (TPC) field on a secondary carrier through a physical downlink control channel (PDCCH); receiving data indicated by the PDCCH; and transmitting ACK/NACK for the data, wherein the ACK/NACK is transmitted using a PUCCH resource which is indicated by the value of the TPC field among the plurality of PUCCH resources.