PURPOSE: To accurately detect variation in toner amount varying the relative distance to an electrode or conductor according to the amount of toner nearby the electrode or conductor.

CONSTITUTION: The toner in a developing tank 1 is consumed as a copying process is carried on. An electrode plate 5 is pressed against a doctor blade 3 when the toner T is much, but leaves the doctor blade 3 when the toner runs out. A toner residue level signal D decreases gradually right before the electrode plate 5 leaves the doctor blade 3, decreases abruptly the moment the electrode plate 5 leaves the blade, and then decreases gradually. For the purpose, the toner residue level signal D is compared with a proper threshold level signal VTH and a toner emptiness signal is inputted to a CPU when the signal D decreases below the signal VTH to display a toner emptiness state and generate a warning, thereby urging an operator to replenish toner. The detection accuracy is improved as compared with conventional devices.