Disclosed is an image forming system, which makes it possible to effectively eject the jam-subsequent paper sheet remaining therein. In the system, a primary abnormal state in which the conveyance operation cannot be continued, and a secondary abnormal state in which an adverse influence is possibly incurred to the image formed on the paper sheet though it is possible to continue the conveyance operation, are defined. In the state that the primary abnormal state or the secondary abnormal state is detected in midcourse of the conveyance operation, when a pre-purge processing for conveying the jam-subsequent paper sheet is implemented so as to eject the jam-subsequent paper sheet onto the ejecting tray, controlling operations are conducted so as to continue the conveyance operation without halting the conveyance operation, even if the secondary abnormal state in respect to the jam-subsequent paper sheet is detected in midcourse of performing the pre-purge processing thereof.