One disclosure of the present invention provides a method for performing a HARQ in a carrier aggregation. The method can comprise a step of receiving setting information with respect to a plurality of cell groups for setting a HARQ ACK/NACK, wherein each of the plurality of cell groups can comprise one or a plurality of serving cells. The method may comprise the steps of: allocating a number of bits of the HARQ ACK/NACK to cell group units, in accordance with the setting information; and transmitting the HARQ ACK/NACK to a relevant cell group in accordance with the number of bits of the HARQ ACK/NACK that are allocated.