An image data output processing apparatus of the present invention, in a case where a reference document is duplex, causes a storage process section, in a storage mode, to store, at the time of storage, a DocID indicative of the reference document so as to correspond to an ID indicative of respective document images on front and back sides of the reference document. In a matching mode, in a case where a matching document is duplex, (i) an ID similar to that of the reference document and (ii) a corresponding DocID are extracted for each of the document images on the front side and the back side of the matching document, so as to create candidate lists for front and back sides of the matching document. If first candidates in the candidate lists correspond to an identical DocID, it is determined that the images are similar to each other, and if the first candidates do not correspond to the identical DocID, a determination result is corrected to a reference document that corresponds to the identical DocID. Thus, it is possible to realize an image data output process that can reliably control an output process, in the case where input image data is of a duplex document and includes a document image in which its output process is regulated.