1. A white line detection apparatus comprising: an extraction device that extracts an image of a white line detection processing area set within a road image captured by an image-capturing device; a maximum value estimating device that estimates a maximum value among pixel values constituting the image of a road surface over the area other than a white line based upon pixel values corresponding to the image extracted by the extraction device; a contrast reducing device that reduces the contrast of an image corresponding to pixel values equal to or smaller than the maximum value among pixel values constituting the image of the road surface over the area other than the white line, which has been estimated by the maximum value estimating device; and a detection device that detects the white line based upon the image whose contrast has been lowered by the contrast reducing device.