Disclosed are a method by which a first device transmits a message in a wireless communication system supporting sidelink, and a device therefor according to various embodiments. The method comprises: a step in which the first device receives a first safety message from a second device; a step of evaluating a collision risk on the basis of the first safety message and generating first event information on the basis of the collision risk evaluation; and a step of transmitting a threat notification message (TNM) including identification information of the second device and the first event information, wherein when the first safety message is a personal safety message (PSM), the threat notification message further includes information on a first risk region determined in response to the first event information.