An image forming apparatus which is capable of efficiently registering apparatuses in a slave apparatus list. The image forming apparatus has a partner apparatus list as the slave apparatus list which stores information on the image forming apparatuses as candidate slave apparatuses so as to select a slave apparatus for use in executing cascade copy. When the image forming apparatus is selected as a master apparatus, a CPU thereof selects a slave apparatus from the image forming apparatuses registered in the partner apparatus list. When the image forming apparatus is selected as a slave apparatus, the CPU determine whether or not the information indicative of the copy mode has been received from a master apparatus. If the information indicative of cascade copy has been received, information on the master apparatus is stored in the partner apparatus list.