With an image forming device, with respect to a set image density S (default mode), the modes of: increasing or decreasing the variation amount (slope) of the set image density in accordance with a usage amount of a developing device (m1, m2); increasing or decreasing the limit value (m1, m2); changing the initial value (usage amount=0); not changing the setting in accordance with the usage amount (m4); and changing, for example, the initial value but not changing the setting in accordance with the usage amount (m5); etc., are provided as operation modes that differ from an operation mode accommodating a genuine product and are enabled to be selected by a user when a toner cartridge containing a toner that is a product other than a genuine product is mounted.