PURPOSE: To prevent a blocking phenomenon of toner, by attaching a slender permanent magnet at the outside of a hopper-shaped magnetic toner tank having a guide plate so as to have an interference with the rotary magnetic field of a permanent roll at the lower opening part of the toner tank.

CONSTITUTION: A hopper-shaped toner tank 4 having a guide plate 5 is set opposite to a developing sleeve 2 containing a permanent roll 3 which turns internally. The magnetic toner supplied to the sleeve 2 is controlled by a doctor plate 6. A permanent magnet 10 having a length almost corresponding to the length of the magnetic pole of the roll 3 is attached at the outside of the toner bottle 4b of the tank 4. With rotations of the roll 3, the rotary magnetic field and the magnetic field of the magnet 10 have the interference to each other. This prevents the accumulation of toner or the blocking and bridging phenomenon of the toner.