A method for supporting a secondary cell (SCell) addition in a radio access system that supports a carrier aggregation (CA), the method is performed by a primary cell (PCell) of a first timing advanced group (TAG) and includes acquiring, from a SCell of a second TAG, random access channel (RACH) resource information related to a random access procedure, wherein the random access procedure is to be performed at the SCell in order to add the SCell to the first TAG; transmitting, to a user equipment (UE), SCell related information, wherein the SCell related information includes a cell identifier of the SCell and the RACH resource information; and receiving, from the UE, a message reporting that the random access procedure at the SCell has failed, wherein the RACH resource information includes RACH parameters required to perform the random access procedure, and wherein a timing advanced value of the first TAG is different from a timing advanced value of the second TAG.