PURPOSE: To perfectly separate developer adhering to a developing sleeve by setting a bias voltage in an earthing state in the case of performing the bristle cutting action of the developing sleeve and making magnetic field act to the developer on the developing sleeve.

CONSTITUTION: This device is provided with a plate-like magnetic substance 11 which is arranged close to developer carriers 7y, 7m and 7c and between a developer regulating means 10 and developer stirring rotors 8y, 8m and 8c in order to make the magnetic field act to the developer, and bias voltage control means 12, 13a and 13b, etc., which impress the bias voltage on the carriers 7y, 7m and 7c in the case of performing development and set the bias voltage in the earthing state in the case of performing the bristle cutting action. Then, the bias voltage is set in the earthing state in the case of performing the bristle cutting action, and the developer adhering to the carriers 7y, 7m and 7c is made easy to be separated by receiving the effect of the magnetic field of the magnetic substance 11. Thus, the developer is perfectly separated from the carriers 7y, 7m and 7c because it is aided also by centrifugal force accompanied with reverse rotation when it passes near a magnetic shielding member 6.