An image forming apparatus to which an external apparatus is connected includes an image forming unit configured to form an image, and a power supply device configured to supply power to the image forming apparatus and the external apparatus. In a case where a value of an alternating current voltage input to the power supply device is a first value, the power supply device supplies first power to the external apparatus. In a case where the value of the alternating current voltage is a second value smaller than the first value, the power supply device supplies second power smaller than the first power to the external apparatus.