An image forming system identifies a user's operation for changing a set value related to a processing of the image forming apparatus or the post-processing apparatus. When the set value is changed during a continuous output operation of the image forming system, the image forming system causes the image forming apparatus and the post-processing apparatus to continue the continuous output operation while reflecting the changed set value. The image forming system Detects a predetermined change related to the processing by referring back to a time when the user's operation is identified, and causes the image forming apparatus and the post-processing apparatus to re-output papers that have been output from a time when the predetermined change occurred to a time when the changed set value was reflected.