A collision avoidance system includes: a radar that detects an object that is located behind a vehicle and that detects a distance to the object; a plurality of ultrasonic sensors, each of which detects the object and detects a distance to the object, the plurality of ultrasonic sensors respectively detect different detection areas; an approaching object detection unit that detects an approaching object that approaches the vehicle from among the objects; a screen estimation unit that estimates that there is a screen that blocks an approach to the vehicle from behind; and a control unit that, when the approaching object has been detected, executes driving assistance for avoiding a collision with the approaching object, and, when a distance to the approaching object is larger by a predetermined value or more than a distance to the screen, restricts or prohibits execution of the driving assistance.