PURPOSE: To surely take out a remaining developer by directing the part, where the magnetic force is minimum, of a magnet roll downward approximately vertically and providing an aperture part on a developing vessel under the magnet roll and providing the aperture part with a cover.

CONSTITUTION: A developing device 17 is attachable to and detachable from a printer and is provided with a detaining piece 15 which sets and fixed the device 16 to the frame of the printer, and a magnet roll 11 constituted in a developing roll 5 is fixed to the developing vessel 1, and the part where the magnetic force is minimum of the magnet roll 11 is directed downward approximately vertically as shown by an arrow A. A cover 9 is provided under the developing roll 5 to cover its overall length, and a step 10b is formed in the peripheral edge of the cover 9, and a step 10a is formed in the peripheral edge of an aperture part 1a on the bottom of the vessel 1 correspondingly to the step 10a, and these steps are combined to form a labyrinty structure 10 between the cover 9 and the peripheral edge of the aperture part, thereby completely preventing the leak of the toner. Thus, the remaining developer stuck onto the developing roll is surely dropped and discharged through the aperture part on the bottom of the vessel by only rotating the sleeve.