The distance to a target vehicle is calculated. To achieve this, a target vehicle traveling ahead is imaged and it is determined to what vehicle group, such as a light-duty vehicle group, standard passenger car group or heavy-duty vehicle group, the image of the target vehicle belongs. Representative vehicle widths are stored in a vehicle group table on a per-vehicle-group basis. The distance from one's own vehicle to a target vehicle is calculated using the vehicle width that corresponds to the vehicle group decided.