One embodiment of the present disclosure provides a method by which a first apparatus transmits a sidelink-synchronization signal block (SL-SSB). The method is characterized in comprising the steps of: determining a first resource considered that a first SL-SSB is transmittable; and transmitting the first SL-SSB to a second apparatus based on the first resource. Accordingly, apparatuses based on V2X communication in wireless communication system may efficiently relay or transmit and receive SL-SSB based on a slot format.