PURPOSE: To obtain a vehicle-to-vehicle distance detecting device which is reduced in calculation quantity and can stably detect the distance between vehicles.

CONSTITUTION: An image processor 11 processes the front road image from a video camera 10 and detects the distance to a vehicle, and a warning level judgement means 12 judges the warning level from the detected value and the detected value of a vehicle speed sensor 13 to actuates a warning device 14. At this time, the image processor 11 detects the horizontal edge showing the position of the precedent vehicle in the road image, calculates the vehicle distance from the position to its own vehicle, and detects two vertical edges showing the width of the precedent vehicle and its width, and detects a horizontal edge and two vertical edges in a newly inputted front road image with the horizontal edge and vertical edges, thereby detecting the distance to the precedent vehicle while tracking and detecting the precedent vehicle according to the vertical edge width and the calculated width of the vehicle. Consequently, the calculation quantity is decreased and the stable detection for the distance to the vehicle is enabled.