PURPOSE: To obtain an image free of image unevenness, high in image density, and low in fog, by inserting an impedance device between a bias source and a development electrode and setting the frequency of the bias voltage to a given value.

CONSTITUTION: During operation of an electrophotographic apparatus, developer resistance drops instantaneously due to temporary developer compression, contact, or the like, dielectric breakdown occurs temporarily, and the conditions where the bias voltage is not applied to the developing electrode for an instant occur, resulting in causing image unevenness due to temporary bias voltage drop when the bias to be applied is of DC. Therefore, power source 45 for applying low frequency AC bias voltage to sleeves 36A, 36B and protective resistor 46 as an impedance device are installed to form a good image and image unevenness is prevented by setting f<1/2t, where f is the AC frequency to be applied and t is the maximal time of the bias voltage leak, and retaining the effect of bias voltage even in bias voltage leak.