An image forming system capable of shortening the time required to process configuration information on an image forming system and shortening communication time. When any of sheet processing apparatuses is activated before activation of an image forming apparatus, a controller of a sheet processing apparatus acting as a sub-manager configures a sub-system consisting of sheet processing apparatuses connected to a communication network and creates configuration data on the sub-system. When recognizing that the image forming apparatus is newly connected to the network, the sub-manager apparatus notifies the image forming apparatus of the already created configuration data based on which a controller of the image forming apparatus creates configuration data on the system and stores it into a RAM.