In order to develop an automatic driving system, it needs to examine the effect of cars in a line on the flow around a car. The related phenomenon is slip stream where the drag force of a car is reduced. The objective of the research is to clarify the drag force on car models in a line arranged linearly at a certain inter-vehicle distance. Therefore, the drag force acting on the models in a wind tunnel was measured by using a load cell. In the experiment, we compared with the effects of attaching or not attaching skirts on a car. Drag force reduction of the following car model was observed only at very short inter-vehicle distances, and the drag force increased when the inter-vehicle distance increased.