An image forming apparatus with a sorter which is capable of performing image formation even during the implementation of a post-process. The image forming apparatus includes the sorter which has a non-sort bin and sort bins for receiving sheets each formed with an image, and a post-processor for performing the post-process on sheets received by the sort bins. When an interruption mode is designated during the post-process, the image formation is permitted while the post-process is continuously performed. Further, the sorter is controlled to discharge a sheet formed with an image into the non-sort bin. Thus, the image formation can be performed concurrently with post-process. Since the sheet formed with the image is discharged into the non-sort bin which is adapted not to receive sheets to be subjected to the post process, a jam will not occur even if an error occurs during the post-process.