A method for enabling a first base station to add a secondary cell, S cell, to carrier aggregation, CA, in a radio access system that supports the CA, the method is performed by the first base station and comprising: receiving a cell measurement report message on neighbor cells from a user equipment, UE; characterized by: acquiring random access channel, RACH, information related to a random access procedure which will be performed by a second base station of neighbor cells; transmitting S cell information to a user equipment, the S cell information including a cell identifier of the second base station and the RACH information; receiving (S1370a, 1370b) a RACH result report message from the user equipment, the RACH result report message indicating whether the random access procedure with the second base station has been successfully performed or not; and when the random access procedure has been succesfully performed, transmitting S cell indication information to the second base station, the S cell indication information indicating that the second base station will be operated as the S cell, wherein the S cell indication information includes a cell identifier of the first base station and UE identifier of the user equipment, and wherein the RACH information includes resource allocation information indicating a resource region of the second base station in which the random access procedure will be performed, and RACH parameters required to generate RACH preamble which will be used for the random access procedure. receiving a cell measurement report message on neighbor cells from a user equipment, UE; characterized by: acquiring random access channel, RACH, information related to a random access procedure which will be performed by a second base station of neighbor cells; transmitting S cell information to a user equipment, the S cell information including a cell identifier of the second base station and the RACH information; receiving (S1370a, 1370b) a RACH result report message from the user equipment, the RACH result report message indicating whether the random access procedure with the second base station has been successfully performed or not; and when the random access procedure has been succesfully performed, transmitting S cell indication information to the second base station, the S cell indication information indicating that the second base station will be operated as the S cell, wherein the S cell indication information includes a cell identifier of the first base station and UE identifier of the user equipment, and wherein the RACH information includes resource allocation information indicating a resource region of the second base station in which the random access procedure will be performed, and RACH parameters required to generate RACH preamble which will be used for the random access procedure. acquiring random access channel, RACH, information related to a random access procedure which will be performed by a second base station of neighbor cells; transmitting S cell information to a user equipment, the S cell information including a cell identifier of the second base station and the RACH information; receiving (S1370a, 1370b) a RACH result report message from the user equipment, the RACH result report message indicating whether the random access procedure with the second base station has been successfully performed or not; and when the random access procedure has been succesfully performed, transmitting S cell indication information to the second base station, the S cell indication information indicating that the second base station will be operated as the S cell, wherein the S cell indication information includes a cell identifier of the first base station and UE identifier of the user equipment, and wherein the RACH information includes resource allocation information indicating a resource region of the second base station in which the random access procedure will be performed, and RACH parameters required to generate RACH preamble which will be used for the random access procedure.