The objective of this study is to reduce rear-end vehicle collisions caused by human errors. In order to achieve this objective, we studied a technique to detect preceding vehicles with image processing using a monocular in-vehicle camera and a computer. As an image characteristic for vehicle detection, we adopt the shadow underneath vehicle. Furthermore, in order to detect the shadow with high precision, we study shadow tracking. In this paper, we describe a technique for vehicle tracking on the basis of the shadow underneath vehicle.