Method and device for transmitting ack/nack in carrier aggregating system

The present invention relates to a method of transmitting an acknowledgement/not-acknowledgement (ACK/NACK) and a device for using such a method. According to the present invention, a cell-specific downlink-uplink setting for one of a plurality of serving cells and a reference UL-DL setting is received, an effective downlink sub-frame capable of actually receiving a data unit from one serving cell is determined based on the settings, and only an ACK/NACK for the effective downlink sub-frame is configured as a bit string and fed back.