The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting a MAC PDU on SL-DCH in a D2D communication system, the method comprising: configuring a first counter to count a number of transmissions within a transmission period; configuring a second counter to count a number of transmission periods; keeping a Medium Access Control (MAC) Protocol Data Unit (PDU) in a MAC entity until the first counter reaches a first maximum value and the second counter reaches a second maximum value; and transmitting the MAC PDU.