To provide a rear side vehicle monitoring device capable of surely detecting a following vehicle with the danger of colliding with a present vehicle from behind and demonstrating a sufficiently highly reliable monitoring function.

The traveling track of the present vehicle is estimated from a car speed and a yaw rate (indicated by a thick solid line), a present vehicle lane is imagined from the traveling track (indicated by a chain line) and the vehicle A traveling on the present vehicle lane is selected from rear side vehicles A and B detected by a laser radar. The vehicle A is considered as the following vehicle with the danger of colliding with the present vehicle from behind and an abnormal approach to the present vehicle is monitored.