To determine authenticity of a solid body simply and precisely, a reference area of a paper sheet which is genuine is optically read from two different directions, and the image is registered as a reference image. A check area of a paper sheet subjected to the authenticity determination, including the reference area and having a size larger than the reference area, is read from two different directions with a scanner, and data on a partial area having the same size as the reference area are extracted from each set of check data collected by the reading. For a set consisting of the reference image and the check image optically read from the same direction, the value of the correlation with the reference image is repetitively calculated by the normalized correlation method while the partial area is shifted within the check area. The maximum correlation value and the normalized score of the maximum correlation value are compared with respective thresholds to determine the authenticity of the paper sheet. If the paper sheet is determined to be “genuine” for the authenticity determination of each set, the paper sheet subjected to the authenticity determination is finally determined to be “genuine.”