Disclosed is a method of transmitting/receiving a signal by a transmission device in a mobile communication system. The method includes: receiving information related to synchronization signals from a Base Station (BS); transmitting a first synchronization signal based on the received information; and receiving a second synchronization signal corresponding to the first synchronization signal based on the received information. When a UE cannot receive a synchronization signal from the BS or is located beyond a coverage of the BS, or when the BS cannot operate due to an emergency or disaster situation, UEs can configure self synchronization therebetween in a predetermined area without any assistance from the BS and thus broadcast and unicast transmission are possible between the UEs based on the synchronization.