There is provided an image forming apparatus that is capable of preventing input errors and eliminating the necessity of complicated input operations in making settings for a sheet mixing operation in which color image output sheets and black-and-white image output sheets are mixed into one group. A stacker tray stores sheets with color images formed thereon by a color MFP and is equipped with a storage device, into and from which information relating to the sheets can be written and read. A black-and-white MFP operates in response to attachment of the stacker tray to an inserter attached to the black-and-white MFP, to execute an image formation job by providing control such that the sheets with color images formed thereon stored in the stacker tray and sheets with black-and-white images formed thereon by the black-and-white MFP are mixed according to the information stored in the storage device of the stacker tray.