To earlier detect the abnormal detection of white line at a branch section.

A window is set for image data photographed by a camera 1, and possible coordinates of a white line are calculated for each window (step S1-S4), and variable parameters containing a pitch angle of vehicle are estimated based on this cross point coordinate of the white line (step S5). In the case the deviation of pitch angle obtained by subtracting the average value of pitch angle DAVE from the estimated pitch angle D is positive, a nose dive counter CND is incremented by '1', and in the case the deviation of pitch angle is negative, the nose dive counter CND is reset to zero value (step S6), and in the case the nose dive counter CND exceeds a threshold NDCNT0, such a decision is made that the nose dive is abnormal, i.e. the detection of white line is abnormal (step S), and an exception operating flag F is set so as F=1, and the parameters stored in a memory 3 are reset (step S8).