Where a toner cartridge other than a genuine product is mounted, the image forming device stops displaying information regarding the usage amount such as the residual amount of toner, etc., and causes the display/input device of the host device to display a screen including information regarding an operation mode and a toner cartridge other than a genuine product. In addition, the image forming device not only stops displaying the residual amount of toner, but also causes the display/input device of the host device to display a screen including information regarding the number of sheets of paper already printed, which is measured or metered from the mounting of a toner cartridge, and the printing coverage thereof.