Disclosed is a method for monitoring a physical downlink control channel (PDCCH) in order to enable communication over a plurality of cells in a mobile communication system, including acquiring a cell index for a cell, wherein the cell index is user equipment (UE)-specific and is informed based on radio resource control (RRC) signaling, monitoring a first PDCCH on a common search space that is for system information and is defined on a single cell for the plurality of cells, monitoring a second PDCCH on a dedicated search space defined for the cell, wherein the cell is one among the plurality of cells, and the dedicated search space is defined for each of the plurality of cells, respectively, and receiving a physical downlink shared channel (PDSCH) on the cell based on the second PDCCH, wherein the cell index is associated with one downlink frequency configured by the RRC signaling.