This description suggests a technique of controlling the timing advance group (TAG) of a cell. In particular, this description suggests a method of managing the TAG in a communication system. The method may include receiving a control signal indicating a cell by user equipment (UE); putting the cell in a first TAG by the UE in response to the control signal, wherein the first TAG includes only at least one secondary cell of which the uplink timing is not decided; performing a random access procedure on the cell in the first TAG by the UE; and putting the cell in a second TAG from the first TAG by the UE.