An electric potential measuring device comprises a detection electrode of a conductive material disposed in opposition to an object and a movable structure comprised of a first solid material portion of a dielectric and a second solid material portion of another dielectric or a conductive material. A charge induced on the detection electrode by electric lines of force from the object is modulated by moving the movable structure with a drive mechanism, to measure an electric potential of the object. An image forming apparatus comprises the electric potential measuring device and an image forming means for performing a control of an image formation by using the electric potential measuring device.