PURPOSE: To apply securely and stably a bias voltage between a developing sleeve and a photosensitive drum, by bringing a conductive brush for the application of the bias voltage into contact with the internal surface of the developing sleeve.

CONSTITUTION: The tip of the conductive brush 25 fitted in the shaft 18 of a magnet roll 14 is energized to contact the internal surface of the nonpicture formation part of the developing sleeve 13, and is rubbed against the internal surface of the developing sleeve 13 stably when the developing sleeve 13 rotates. The shaft 18 of the magnet roll 14 is made of a conductive material and connected to an alternating bias power source through wiring from the shaft end to apply the alternating bias voltage between the developing sleeve 13 and photosensitive drum 3 through the conductive brush 25.