The present invention relates to a wireless communication system. More particularly, the present invention relates to a method for transmitting acknowledgement/negative ACK (ACK/NACK) in a wireless communication system which supports carrier aggregation, and to an apparatus for the method. A method in which a terminal transmits ACK/NACK in a wireless communication system that supports carrier aggregation comprises the following steps: receiving one or more physical downlink shared channels (PDSCHs); and transmitting ACK/NACK for said one or more PDSCHs via a physical uplink control channel (PUCCH). A PUCCH format for transmitting ACK/NACK is selected by taking the type of the carrier in which said one or more PDSCHs are received into account.