When an image forming apparatus receives print data to which page quantity information has been added from an external apparatus, it decides by which emulation among a plurality of types of emulations to interpret the print data, and outputs the print data emulation-processed depending on the decision to an engine which forms an image for each page. Further, when it is determined that the page quantity of the print data outputted to the engine is larger than the page quantity indicated by the page quantity information, the apparatus instructs the engine to suspend image formation.