To extract traffic lane lines on road surfaces as connected polylines.

Road surfaces are imaged with a video camera as an operator drives on the roads, and the position coordinates of each point imaged are obtained using a GPS or the like. A computer transforms each frame image of this moving image to create orthophotos seen from immediately above. Based on the position coordinates of the imaged points, the orthophotos are arranged on a path traveled, so as to create a connected image, from which traffic lane lines are extracted by image processing. Markers where intervals d22 between traffic lane lines perpendicular to the path NP 22 when imaging are not greater than a predetermined value and markers whose paths are biased relative to the lane lines are deleted as false recognitions. In this way, the false recognitions are deleted according to distances between the lane lines and the paths or other traffic lane lines, and markers defining the boundaries of the lane lines are specified and connected together, whereby the traffic lane lines can be extracted as polylines.