In a serial tandem image forming system having a plurality of image forming apparatuses, a double-side printing job can be performed in a way according to the needs of users by taking into consideration an adjustment operation. The serial tandem image forming system includes an upstream apparatus 200 and a downstream apparatus 300, as the image forming apparatuses, each of which is capable of forming images on both sides of a sheet. When receiving a double-side printing job while one of the image forming apparatuses is performing the adjustment operation, a control unit 260 displays information on a manipulation display 500 which prompts a user to select whether to immediately start the job by performing a single machine double-side printing process with the other image forming apparatus, or start the job by shifting to a tandem mode and performing a serial double-side printing process after the adjustment operation has finished.