An image forming system includes: a sheet feeder; an image former that forms an image on a paper sheet; a conveyor that conveys a paper sheet to a sheet ejection destination; a switcher that switches sheet ejection destinations; a reader that optically reads a paper sheet, and outputs a read image; an abnormal sheet detector that detects an abnormal paper sheet by analyzing the read image; a storage; a display; and a hardware processor that performs: ejection destination switch control to control the switcher to eject an abnormal paper sheet and a succeeding paper sheet to a second ejection destination different from a regular ejection destination; storage control, for each of the abnormal paper sheet and the succeeding paper sheet, to associate sheet information, the read image, and a stacking order with one another, and store them into the storage; and display control to cause the display to display them.