The present invention relates to a method and a device by which first user equipment (UE) that is located out of the range of a group communication service determines a relay through a proximity service (ProSe) in a wireless communication system. Particularly, the method comprises the steps of: receiving traffic transmitted by second UE which already relays group communication; determining whether the traffic is information bound to group communication; determining relay UE by using a discovery procedure for a relay for the group communication, when the traffic is not recognized as the information bound to the group communication; and determining the second UE as the relay UE for the group communication, when the traffic is recognized as the information bound to the group communication.