In an autonomous vehicle, position of a planar obstacle such as a wall or a fence having apertures is determined. A horizontal plane is scanned with a laser range tinder so as to acquire positional coordinates of a plurality of scanning points. An element vector is formed with each two scanning points so that one acquired former is used as a start point and the other acquired later as an end point of the vector. A plurality of continuous element vectors which satisfy predetermined conditions is selected among the element vectors, and a scanning segment vector is formed by composition of the selected element vectors. When a length of the scanning segment vector is equal to or longer than a predetermined length, it is possible to recognize that a planar obstacle exists along the scanning segment vector.