PURPOSE: To maintain specified recording density and to suppress the increase of a device cost by identifying whether the toner of a freely attachable and detachable developing cartridge is black or color by an identifying means and changing over automatically a toner replenishing method in the stage of using the black toner and in the stage of using the color toner.

CONSTITUTION: The freely attachable and detachable cartridge-shaped developing device is provided with identifying marks MK1, MK2 which are colored to the black or white according to the color of the toner to be used. Photosensors PS1, PS2 consisting of light emitting diodes and phototransistor detect the reflected light of the marks MK1, MK2. The MK1 is black and the MK2 is white and therefore the sensor PS1 turns off and the PS2 turns on when a developing unit contg. the toner of, for example, a sepia color is mounted. The output of an AND gate AN3 rises to an H and a display device LE3 is lighted up via a driver DV3. The prescribed quantity of the toner is replenished according to the color thereof.