A control apparatus controls at least one of the first image forming apparatus and the second image forming apparatus connected downstream of the first image forming apparatus so that when switching from a first mode to a second mode, if the second image forming apparatus is unable to perform image forming on a second side of a sheet whose first side has undergone image forming in the second image forming apparatus before the second image forming apparatus performs image forming on a second side of a sheet whose first side has undergone image forming in the first image forming apparatus, an order of the sheet whose first and second sides undergo image forming in the second image forming apparatus and the sheet whose first side undergoes image forming in the first image forming apparatus and whose second side undergoes image forming in the second image forming apparatus is not swapped.