An image forming apparatus, to which an accessory device is connectable, includes a control unit configured to control the image forming apparatus to start a preparation operation in which an image forming unit can form an image in response to a power supply, and a storage unit configured to store a connecting state between the image forming apparatus and the accessory device. The control unit determines, in response to the start of the power supply, whether the accessory device has been connected to the image forming apparatus before the power supply is stopped, with reference to information stored in the storage unit. Then, the control unit controls the image forming apparatus to start the preparation operation without waiting for the receiving of the information about the accessory device therefrom if the accessory device has not been connected to the image forming apparatus.