1. A driving assistance system comprising: an image sensing device, installed in a vehicle, for obtaining an image of an area behind and next to the present vehicle, wherein the area includes a lane other than the present lane on which the present vehicle runs; a display device for displaying the image obtained using the image sensing device in a manner such that the image is visible to a driver of the present vehicle; and a mark display device for superimposing a mark on the image, wherein the mark indicates a degree of danger with respect to another vehicle running on the other lane when the present vehicle enters the other lane.