To improve measurement accuracy for a distance between vehicles by arranging three or more identification means at a rear face of a vehicle body of a preceding vehicle and measuring a relative position to the preceding vehicle on the basis of a binary image.

Three IR(infrared ray)-LEDs 2 are arranged at position of angles of an isosceles triangle at a rear face 1a of a body of a preceding vehicle 1. An IR-camera 4 for photographing the rear face of the body of the preceding vehicle 1 and an image-processing device are set at a following vehicle 3. The image-processing device converts an input image from the IR-camera 4 to a binary image of a brightness level. The IR-LEDs 2 are divided with the rear face 1a of the preceding vehicle 1 at the background, so that three bright pints A1-A3 are given to the binary image. In order to obtain a coordinate position of each bright point A1-A3, a histogram in an (x) direction and a (y) direction is formed from the binary image. The coordinate position of each bright point A1-A3 is determined from the histogram. A distance between bright points is calculated from the coordinate positions, and a distance between the vehicles is obtained from the calculated values, a focal length of a camera lens and an actual distance between bright points.