An image forming apparatus including an image bearing member, a charging
member, and a measuring member. The image forming member is configured to
bear an image. The charging member is configured to charge a surface of
the image bearing member when a direct current bias voltage is added to
at least an alternating bias voltage which is applied to the charging
member, and has a frequency fc of the alternating bias voltage. The
measuring member is configured to measure a potential of the surface of
the image bearing member, and has a drive frequency fvsen. The
relationship of the frequency fc and the drive frequency fvsen is
expressed as fc.noteq.fvsen.times.n and fc.times.n.noteq.fvsen.