The present specification proposes a scheme for controlling a deactivation timer of a cell. More specifically, the present specification proposes a method of controlling a deactivation timer of a cell in a mobile communication system. The method includes: configuring, at a user equipment (UE), a first deactivation timer of a first cell and a second deactivation timer of a second cell, wherein the first cell and the second cell belong to a same timing advance group (TAG); if the first deactivation timer of first cell expires, checking, at the UE, whether uplink timing of the same TAG is required to be maintained; and if the uplink timing of the same TAG is required to be maintained, restarting, at the UE, the first deactivation timer of the first cell.