This paper presents a vehicle detection method which can divide overlap vehicles. The proposed method detects vehicles by finding vertical planes above a road plane. At first, two horizontal line segments adjacent vertically are detected in the present frame. Then, corresponding line segments are searched in the previous frame by a correlation matching. If the motion of these horizontal line segments satisfy the constraint of the vertical plane, it is estimated that a plane between these line segments is a part of vertical surfaces on a vehicle. These vertical planes are merged if horizontal line segments on these planes satisfy the motion constraint of the vertical plane. Each merged region provides an individual vehicle. The proposed method can divide overlap vehicles because the back surface on each vehicle is detected as the different vertical planes respectively. Experimental results for real road scenes under a traffic jam show the effectiveness of the proposed method.