PURPOSE: To obtain an invariably stable picture even at the time of variation in the humidity of a developer or an increase of the number of copies by detecting the resistance value of the developer and holding it invariably constant.

CONSTITUTION: An AC power source 4 connected to a capacitor 3 and a DC power source 5 for biasing are connected to a magnetic brush sleeve 2 together. A resistance detector 9 detects the resistance on a sleeve 1 and a developer in a carrier 8 and is connected to the controller 10 for the power source 4. The device 10 is operated by the detector 9 to control the voltage or frequency of the power source 4 so that the resistance value of the developer on the sleeve 2 is constant. For example, the voltage of the power source 4 is 1W2.5V and pulse width is 0.1msec; and duty is 30% and the frequency of repetition is 2kHz. In this case, when the resistance value of the developer is held constant, stable development density is obtained regardless of variation in relative humidity and an increase in the number of copies.