A thick-frame attention-calling mark 49 that is approximately rectangular is added to the following vehicle 48 to surround the outline thereof. Instead of such a rectangular thick frame, the attention-calling mark 49 may have any of a variety of shapes and signs. For example, an attention-calling mark 49′ of a modified example illustrated in FIG. 15B is formed as a round thick frame. An attention-calling mark 49″ illustrated in FIG. 15C is formed as an elliptic thick frame with a part thereof eliminated to surround the following vehicle 48 on a road.