Entwicklungseinheit mit wirkungsvollem Austausch des Entwicklungspulvers

A developing unit comprises a magnet roller, a stirring roller, a carrying screw, a toner density sensor, and a sensor level adjusting device. When feeding or exhausting the developing powder, the rollers and screw are driven at a slower or faster rotary speed than the speed given when developing an image. After the supply of the developing powder is terminated, the rotary speed of the rollers and the screw is changed to the speed given for developing an image. After a certain time is elapsed, the adjusting device serves to start the level adjustment when the sensor produces a stable output.