A relative position calculator calculates relative position and attitude of a vehicle based on images captured by an image capturing apparatus on the vehicle. An absolute position calculator extracts a marker from an image captured by the image capturing apparatus, and calculates absolute position and attitude of the vehicle, based on position and attitude of the one extracted marker. A corrector calculates the corrected position and attitude, not using the absolute position and attitude calculated based on the position and attitude of the marker when a difference or ratio of an apparent height and width of the marker in the image is equal to or smaller than a threshold, but using the absolute position and attitude calculated based on the position and attitude of the marker when the difference or ratio is larger than the threshold.