The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for allocating priorities to a logical channel group implicitly in a D2D communication system, the method comprising: receiving information including N numbers of priority lists in order of increasing logical channel group identity from an eNB, associating each of the N numbers of priority lists with each of the N numbers of logical channel groups in an order corresponding to a sequential order of the information, when a sidelink logical channel with a first priority is configured, allocating the sidelink logical channel with the first priority to a logical channel group associated with the first priority.