There is provided an image forming apparatus which can output a job with a higher priority level in a timely manner without processing other jobs first, when a container has been attached to the image forming apparatus. Images on sheets are formed in accordance with an image forming job by a color MFP 104. The sheets on which the images have been formed is conveyed to a stacker tray 1207 detachably attached to an inserter 108 attached to the black-and-white MFP 105. Storage of the sheets on which the images have been formed in the stacker tray 1207 is controlled to selectively inhibit or allow the storage according to the priority level of the image forming job by a CPU 1805 of the color MFP 104.