PURPOSE: To reduce the cost and to miniaturize a processing cartridge.
CONSTITUTION: The image forming device consists of a constitution where the processing cartridge K is freely attachable/detachable to a device main body 1. A detecting means (photosensors 27a and 27b) are provided to detect the number of rotation (number of image formation) of a photosensitive drum 10. Non disappearing ink (display means) 28 is provided on the device main body 1 side having its position varied according to the number of rotation of the photosensitive drum 10 detected by the detection means and to display residue quantity of toner in the processing cartridge K. Therefore, since the number of rotation of the photosensitive drum 10 (residue toner quantity) is detected on the device main body 1 side and the residue quantity of the toner is displayed to the processing cartridge K side, there is no need to provide a part to detect the residue toner quantity on the processing cartridge K side. The number of parts of the processing cartridge K, which are exchange parts, can be reduced bringing down the cost and miniaturizing the device.