Provided are an apparatus and a method for executing an uplink hybrid automatic repeat request (HARQ) in a carrier aggregation system. The method transmits data to a base station from an uplink subframe of a second cell, and receives, from the base station, an acknowledgement/not-acknowledgement (ACK/NACK) or scheduling information for the data from a downlink subframe of a first cell, wherein if the timing relationship between the uplink subframe of the second cell and the downlink subframe of the first cell does not match the timing relationship for when the first cell is being used exclusively, then the scheduling information is received from the downlink subframe of the first cell.