An image forming apparatus having a potential sensor for measuring a potential at the surface of a photoreceptor drum; a voltage generation circuit for generating a voltage lower than a voltage to be actually induced at the surface of the photoreceptor drum in the course of image formation; switching means for switching a condition so that the photoreceptor is connected to the voltage generation circuit or earthed; and charger-output control means for obtaining a value of a surface potential of the photoreceptor measured by the potential sensor at the time when a voltage form the voltage generation circuit is applied to the photoreceptor by operating the switching means, estimating by arithmetic operation, on the basis of the measured value thus obtained, a value to be measured by the potential sensor when a high voltage is actually induced at the photoreceptor surface in the course of the image formation, and controlling an output from the charger based on the value estimated by the above arithmetic operation when a voltage is applied to the photoreceptor surface by the charger prior to image forming operation.