There is provided an image forming apparatus capable of setting up a tab sheet mode-operating and configuring environment in which an effective tab sheet mode output can be obtained by simple selecting and designating operations, A designating device designates a type of sequence of tabs of each of tab sheet bundles set in the sheet feed devices such as cassettes 21a-21d and multi manual feed tray 27 etc. A determining device is responsive to a user's instruction for feeding tab sheets and selection of a combination of one of the sheet feed devices and one of the sheet discharge devices such as face-up discharge tray 2 and face-down discharge tray 3, for determining whether a combination of the selected sheet feed device, the selected sheet discharge device, and the type of sequence of tabs of the tab sheet bundle set in the selected sheet feed device is valid or not.