An image forming system including an image forming apparatus and a post-processing apparatus that performs post-processing on sheets discharged from the image forming apparatus, comprising: feeding means adapted to store a plurality of sheets and successively feeding the sheets; image forming means adapted to form an image on a sheet according to a setting content of an image formation job; stack means (590, 591) adapted to stack sheets processed by the post-processing apparatus; state detection means (951, 952, 953) adapted to detect a predetermined state of the stack means (590, 591), being a state in which a remaining number of sheets receivable by the stack means (590, 591) is restricted; and control means adapted to restrict the feeding of sheets from the feeding means following detection of the predetermined state; characterized by: receivable sheet number determining means adapted to determine, based on the setting content of an image formation job, a number of sheets that the post-processing apparatus can receive from the image forming apparatus after detection of the predetermined state; and conveyed sheet number determining means adapted to determine a number of sheets existing in a sheet conveyance path extending from the feeding means to the stack means (590, 591); and further characterized in that the control means are adapted to restrict a number of sheets fed from the feeding means so that the number of sheets determined to exist in said sheet conveyance path by said conveyed sheet number determining means does not exceed the receivable number of sheets determined by said receivable sheet number determining means. feeding means adapted to store a plurality of sheets and successively feeding the sheets; image forming means adapted to form an image on a sheet according to a setting content of an image formation job; stack means (590, 591) adapted to stack sheets processed by the post-processing apparatus; state detection means (951, 952, 953) adapted to detect a predetermined state of the stack means (590, 591), being a state in which a remaining number of sheets receivable by the stack means (590, 591) is restricted; and control means adapted to restrict the feeding of sheets from the feeding means following detection of the predetermined state; characterized by: receivable sheet number determining means adapted to determine, based on the setting content of an image formation job, a number of sheets that the post-processing apparatus can receive from the image forming apparatus after detection of the predetermined state; and conveyed sheet number determining means adapted to determine a number of sheets existing in a sheet conveyance path extending from the feeding means to the stack means (590, 591); and further characterized in that the control means are adapted to restrict a number of sheets fed from the feeding means so that the number of sheets determined to exist in said sheet conveyance path by said conveyed sheet number determining means does not exceed the receivable number of sheets determined by said receivable sheet number determining means. receivable sheet number determining means adapted to determine, based on the setting content of an image formation job, a number of sheets that the post-processing apparatus can receive from the image forming apparatus after detection of the predetermined state; and conveyed sheet number determining means adapted to determine a number of sheets existing in a sheet conveyance path extending from the feeding means to the stack means (590, 591); and further characterized in that the control means are adapted to restrict a number of sheets fed from the feeding means so that the number of sheets determined to exist in said sheet conveyance path by said conveyed sheet number determining means does not exceed the receivable number of sheets determined by said receivable sheet number determining means.