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 cell with PUCCH resource and a second cell with PUCCH resource; configuring zero or more third cells without PUCCH resource; generating a first HARQ feedback by multiplexing HARQ feedbacks of all HARQ processes of the first cell and third cells associated with the first cell; generating a second HARQ feedback by multiplexing HARQ feedbacks of all HARQ processes of the second cell and third cells associated with the second cell; and transmitting the first HARQ feedback on the first cell with PUCCH resource and the second HARQ feedback on the second cell with PUCCH resource.