One disclosure of the present specification provides a method for performing vehicle to everything (V2X) communication on a sidelink, the method being performed by a V2X device. The method may comprise a step for selecting one among a plurality of synchronization reference sources which can be used to transmit a V2X signal on a sidelink. The method may comprise: a step for performing time synchronization, in order to transmit the V2X signal on the sidelink, on the basis of a downlink signal from an NR-based cell or an L TE-based cell, on the basis of the NR-based cell or L TE-based cell being selected as a synchronization reference source; and a step for transmitting the V2X signal on the sidelink on the basis of the synchronization.