The present invention includes: image capturing means (2) which captures a color image of a road via imaging means (7); area extraction means (3) which extracts areas having a similar color feature value from the captured color image of the road; white balance processing means (5) which performs, for each of the extracted areas, a white balance process of correcting each pixel data in the extracted area so as to reduce the degree of divergence in the level between color components according to a level balance between the color components of the pixel data in the extracted area; and road sign detection means (6) which detects a road sign on the road from the color image subjected to the white balance process. Thus, even if the road illumination state is partially different, it is possible to accurately detect a road sign such as a lane mark from the captured color image of the road via the imaging means such as a camera.