The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting multiplexed HARQ feedbacks in a carrier aggregation system, the method comprising: configuring a first Physical Uplink Control Channel (PUCCH) group and a second PUCCH group, wherein at least two cells are configured with PUCCH resources in each of the first PUCCH group and the second PUCCH group; generating a first Hybrid-ARQ (HARQ) feedback by multiplexing HARQ feedbacks of all HARQ processes of all cells belonging to the first PUCCH group; generating a second HARQ feedback by multiplexing HARQ feedbacks of all HARQ processes of all cells belonging to the second PUCCH group; selecting a first cell among the at least two cells with PUCCH resources in the first PUCCH group; selecting a second cell among the at least two cells with PUCCH resources in the second PUCCH group; and transmitting the first HARQ feedback on the first cell and the second HARQ feedback on the second cell.