1. A method of performing communication by a user equipment (UE) in a wireless communication system, the method comprising:receiving first layer 1 (L1) information related to M (M?N) number of unlicensed band cells (UCells) to which a downlink resource is temporarily set among N number of UCells from an eNB;
receiving second L1 information related to a maximum number of K of UCells that the UE is able to aggregate;
aggregating all of the M number of UCells, when K is no less than M;
aggregating K number of UCells selected by the UE from the N number of UCells based on a predetermined rule, without an indication from the eNB related to the selected K number of UCells, when K is less than M; and
performing communication with the eNB using the aggregated UCells.