Collision avoidance system

A collision avoidance system (100) comprising: a following vehicle data acquisition unit (51) that obtains following vehicle data indicating the relative position and relative speed, relative to a following vehicle (12) traveling behind a vehicle (11); a travel data acquisition unit (52) that obtains travel data indicating travel conditions for the vehicle; a specific state extraction unit (22) that, on the basis of the following vehicle data and the travel data, extracts specific travel data indicating specific travel conditions for the vehicle under which the possibility of collision between the vehicle and the following vehicle is high; a database unit (23) storing a plurality of pieces of specific travel data; a determination unit (24) that, on the basis of the travel data obtained by the travel data acquisition unit and the specific travel data stored in the database unit, determines whether or not there is the possibility of collision between the vehicle and the following vehicle; and an alarm data output unit (25) that outputs alarm data to the following vehicle if a determination has been made by the determination unit that there is the possibility of collision.