An image forming apparatus in which surplus tabbed sheets produced in one set of output can be utilized for the next set of output, without requiring a user to perform a laborious task. In the image forming apparatus, a CPU calculates the number of tabbed sheet groups which will be used for one set of image formation and the number of tabbed sheets which will be used from among the last tabbed sheet group. Among the last tabbed sheet group, surplus tabbed sheets which will be left unused in the one set of image formation are determined. If the number of tabbed sheets is equal to or less than the number of the surplus tabbed sheets, at least one of the surplus tabbed sheets is used to carry out two sets of image formation.