The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for indicating a ciphering indication for a sidelink radio bearer in a D2D communication system, the method comprising: receiving a PDCP SDU when the UE is configured to communicate with one or more other UEs directly; deciding whether to apply ciphering or not for the received PDCP SDU; generating a PDCP data PDU including the received PDCP SDU and a PDCP PDU header including one or more fields for ciphering parameters; and transmitting the PDCP data PDU to the one or more other UEs over PC5 interface, wherein at least one of the one or more fields for ciphering parameters is set to a fixed value when the ciphering is not applied for the received PDCP SDU.