An image forming system is given, in advance, unique setting as a behavior to be performed when an error occurs regarding postprocessing. In a case where the postprocessing becomes inexecutable during the operation of the system, the system either cancels the postprocessing to continue printing (Steps S20, S22) or stops printing to generate an error (Step S18), based on the unique setting. A user arbitrarily decides the unique setting, which enables optimum control of the operation that the system should perform when the postprocessing becomes inexecutable.