Only points on a linear mark image is detected as linear mark characteristic points without influence of surrounding structures and noise, so that reliability is improved on detecting a linear mark. As shown in FIG. 1, the present invention is provided with a white-line inside/outside luminance difference computing means 102, a white-line end edge intensity computing means 103, a white-line internal luminance uniformity computing means 104, and a white-line characteristic integrating means 105. A plurality of characteristics including a difference in luminance between the inside and the outside of a white line, edge intensity, and uniformity of luminance in a white line are integrated to compute a likelihood of a white-line.