A digital copying machine as an example of an image forming apparatus sets a copying operation time period per sheet based on the size of a copying sheet. If the remaining number of sheets corresponding to an expendable in the digital copying machine decreases, time at which this expandable runs out is displayed, and time at which a discharge tray is filled is displayed if the number of sheets to be discharged onto the discharge tray is expected to exceed a fixed value. As a result, an image forming apparatus permitting an operator to smoothly perform processings with the apparatus if a copying operation to a set of documents formed of a plurality of sheets cannot be completed.