A fixed data memory stores data of a normal lane width between lane lines and a narrow lane width between inner guide lines of double white lines. A lane-line candidate setting section detects lane-line detection points on both sides of a driving lane using a captured image, and sets lane-line candidate points on the opposite lane lines at spaces of the widths and therebetween, using the detection points as starting points. A curve approximation processing section sets virtual lines on both sides of the driving lane from a curve approximation equation obtained using the detection and candidate points. A lane-line position setting section obtains dispersions of the candidate points to the left and right virtual lines, and estimates the type of at least one of the left and right virtual lines.