To increase detection accuracy of surface potential of a photoreceptor by increasing the SN ratio of an output signal of a potential sensor even when electrification or development is performed using voltage including alternating voltage.

High frequency noise is prevented from coming into a thin film electrode layer 32 inside a film 31, by forming shielding layers 37a and 37b on the outer surface of the potential sensor 30 and connecting them to ground potential. Thus, the SN ratio of the output signal of a potential sensor is increased, and the detection accuracy of the surface potential of the photoreceptor can be improved.