An image forming system includes an image forming apparatus having an image reading device, and a post-processing apparatus having a manual staple mechanism. The manual staple mechanism is capable of performing staple processing on condition that power is supplied from the image forming apparatus to the post-processing apparatus. The image forming apparatus stops power supply to the post-processing apparatus on condition that a predetermined time has elapsed since reference timing. When the image forming apparatus reads an original with the image reading device, the image forming apparatus determines presence or absence of a staple mark in the original based on a read result. When the image forming apparatus determines that the staple mark is present, the image forming apparatus extends the predetermined time.