An image forming apparatus that permits alteration of print data during execution of a print job for the print data stored in an external apparatus. Upon generation of an interruption job during execution of the print job for the print data, an MFP suspends the print job. If free space in a memory is less than the capacity required for execution of the interruption job, the MFP erases the print data in the memory and carries out the interruption job. When the interruption job is completed, the MFP transmits to another MFP a request for transmission of copy data of the print data and restarts the print job for the copy data of the print data transmitted from the other MFP in response to the transmission request.