A method for matching time synchronization between UEs during Device-to-Device (hereinafter referred to as “D2D”) communication in a radio communication system without involving an Evolved Node B (ENB) is proposed. Using the proposed method, all the UEs can operate to match the time synchronization without malfunction in accordance with a synchronization signal of the ENB even if the UE inside an ENB area and the UE outside the ENB area coexist.