A serial tandem image forming system is described which is capable of inhibiting both the degradation of image quality and the reduction of the productivity. This system consists mainly of the control units of upstream and downstream apparatuses. When a preceding sheet differs from the subsequent sheet in sheet type, a control unit for controlling the image forming system has the subsequent sheet stand by at a paper stop roller of the upstream apparatus, and performs the operation of switching the operating states of the upstream and downstream apparatuses in accordance with the sheet type of the subsequent sheet. Also, the control unit predicts the timing of finishing the switching of the operating state, and then re-starts the conveyance of the subsequent sheet, in advance of finishing the switching of the operating state, by controlling the paper stop roller on the basis of the predicted finishing timing.