In an image data output processing apparatus of the present invention, an image matching section is capable of determining whether a similarity exists between each image of an N-up document and a reference document when input image data is indicative of the N-up document. An output process control section is capable of regulating an output process of each image in accordance with a result of determining whether the similarity exists between each image of the N-up document and the reference document. This allows detecting with high accuracy a document image under regulation on the output process and regulating the output process, when the input image data is indicative of an N-up document and includes the document image under regulation on the output process.