The main body of the image forming system includes an image forming apparatus; an optional apparatus; and a communication unit configured to perform communication between the main body of the image forming apparatus and the optional apparatus. The optional apparatus includes one optional control unit for controlling multiple optional functions, and the main body of the image forming apparatus includes a registration designation unit configured to designate registration of an ID number to the one optional control unit of the optional apparatus by the communication unit to designate registration of an ID number to the one optional control unit of the optional apparatus. The one optional control unit assigns an ID number to each of the multiple optional functions according to the designation from the main body of the image forming apparatus.