Luminous values of pixels of coordinates previously set as an obstacle presence line are inputted to a memory as a one-dimensional signal converted from a two-dimensional image signal taken by a camera. A portion of the line has a luminous value other than a luminous value predicted as a luminous value of a normal road. The portion of the line is then extracted as a vehicle candidate range. Further, based on characteristic of the vehicle such as a vehicle width or a relative speed, presence or absence of the vehicle is determined by removing noise from the extracted vehicle candidate range.