The invention provides an image forming system which includes an image forming device and a post-processor, and complete warming-up of the image forming device in a short time with a limited power supply. The invention further provides an image forming system which includes an image forming device and a post-processor, and enables saving consumption power consumed by the heating section of the post-processor if the heating section is used at long intervals. The invention yet further provides an image forming system which has an image forming device and a bookbinding device, and reduces the maximum consumption power, with a simple structure.