PURPOSE: To measure surface potential in both polarities even in the case of using a potential measuring means for a single polarity by shifting a reference potential impressed on the earth terminal of the potential measuring means to a reversed polarity side to the surface potential to be measured, measuring the surface potential to be measured in the range of both polarities, positive and negative, and calculating the surface potential of a photosensitive body.

CONSTITUTION: A potential to be detected Vin which is detected by an electrometer probe 5 is inputted in an electrometer PCB 9 in specified timing. Since voltages +30V and +5V are respectively impressed on a power source input terminal (Vc) and the earth terminal in the electrometer PCB 9, the potential to be detected Vin is obtained in the range of both polarities, positive and negative. The potential to be detected Vin is reduced by a constant in the electrometer PCB 9 and a detecting potential V0 is obtained. In a microcomputer CPU 11, the detecting potential V0 is converted in V=-200(V0-5)+5 and the surface potential V of the photosensitive body 1 is measured. Thus, the surface potential of the photosensitive body having both polarities is measured by using the electrometer for single polarity.