The present invention improves authentication accuracy. A first ranking among a plurality of reduced registration images is determined based on the similarities of the plurality of reduced registration images with respect to each of the reduced registration images used as a reference. Further, a second ranking among the plurality of reduced registration images is determined based on the similarities of the plurality of reduced registration images with respect to a reduced comparison image. Then, in the case where none of first ranking data has a ranking correlation value with respect to second ranking data equal to or larger than a predetermined threshold, it is determined that authentication has failed.