The invention relates to a method comprising classifying user equipment (UE) cells, to which carrier aggregation is applied, into a plurality of time advance groups. Uplink-time synchronization is managed using the time synchronization of a primary cell (Pcell) in the time synchronization group to which the primary cell belongs. For other time synchronization groups, the uplink time synchronization is managed using the time synchronization of a secondary cell (SCell) that is particularly set in the relevant time synchronization group. Thus, the plurality of time synchronizations can be effectively managed for the UE and a base station (BS).