An image forming system that is capable of preventing obstruction of transmission of a packet to an information processing apparatus from a USB device supporting USBNW due to the shift to the sleep mode. The image forming system includes an information processing apparatus, an image forming apparatus with a sleep mode, and a USB device that is connected to the information processing apparatus through the image forming apparatus and is controlled by the information processing apparatus. The image forming apparatus includes a first determination unit that determines whether the USB device supports USBNW, a second determination unit that determines whether a packet received from the information processing apparatus is processed by a module supporting USBNW, and a control unit that prohibits the image forming apparatus from shifting to the sleep mode when the USB device supports USBNW and when the packet is processed by the module supporting USBNW.