In storing of a document page index of a reference image in association with each hash value calculated for each reference image by the features calculating section, a storage processing section determines whether the number of document page indexes, which have already been stored in association with each hash value, is larger than or equal to a preliminarily setup upper limit value or not. The storage processing section stores the document page index of a reference image in a hash table in association with the hash value calculated by the features calculating section when the number of document page indexes, which have already been stored, is smaller than the upper limit value, or invalidates the hash value when the number of document page indexes, which have already been stored, is larger than or equal to the upper limit value.