Respective operation mode histories are stored in the main body NVM of an image forming device. The main body NVM stores an operation mode just before the turning-off of the power supply. When the power supply of the image forming device is turned on, a CPU discriminates whether a replacement unit having been attached is a genuine type or one other than the genuine type. When an operation mode just before the turning-off of the power supply differs from an operation mode corresponding to the replacement unit having been attached, such a fact is displayed on a UI device. When a user confirms the display on the UI device, the image forming device is controlled by the operation mode corresponding to the replacement unit.