An image forming apparatus capable of suppressing the productivity of image formation from being lowered due to discharge of surplus tabbed sheets in a double-sided printing job in which output sheet bundles are produced by inserting tabbed sheets into each bundle of double-sided printed recording sheets. In a case that output sheet bundles are each produced by setting tabbed sheet bundles each comprised of five tabbed sheets to the image forming apparatus, by inserting one tabbed sheet subsequently to double-sided image formation on five recording sheets, and by performing double-sided image formation on other five recording sheets, four surplus tabbed sheets are discharged subsequently to feeding of the tabbed sheet, instead of discharging the four surplus tabbed sheets after completion of image formation on a second surface of a tenth recording sheet.