The present invention relates to a lane-dividing line detection device and has an object of detecting a lane-dividing line with high accuracy by accurately extracting characteristic points of the lane-dividing line while responding flexibly to road surface conditions. According to the present invention, pixel parts where a brightness variation is larger than a predetermined threshold are extracted from an image picked up by a camera that picks up an area ahead of a vehicle as edge points representing the lane-dividing line on a road surface (step 106). Next, candidates for the lane-dividing line drawn on a road are generated based on the extracted edge points (step 108). Then, the predetermined threshold for the brightness variation used to extract the edge points is changed based on the number of the generated candidates for the lane-dividing line (steps 122 through 130). Specifically, if the number of the candidate lines exceeds a predetermined number, the threshold is changed to a high value so as to increase difficulty in extracting the edge points. On the other hand, if the number of the candidate lines does not reach the predetermined number, the threshold is changed to a low value so as to facilitate the extraction of the edge points.