The invention provides an image formation apparatus having a photosensitive drum, primary and secondary chargers which are controlled by first and second high voltage transformers, respectively, a predischarge charger controlled by a predischarge high voltage transformer, a transfer charger controlled by a transfer high voltage transformer, an exposure lamp controlled by a light control circuit, a developing roller which receives a controlled bias voltage from an AC developing bias circuit, a surface potentiometer, and a microprocessor. In preliminary exposure, an original is exposed to light to form an electrostatic latent image on the photosensitive drum. The potential of this latent image is measured by the surface potentiometer so as to detect the potential of the original. Based on the potential of the original and hence a detected density of an image of the original, image formation conditions such as a development bias voltage, corona charge, and an original exposure voltage are corrected.