PURPOSE: To perform optimum picture density control automatically by detecting the surface of a photosensitive drum where no toner is supplied by a photosensor prior to a copying process, and correcting the sensitivity of the photosensor according to the detected level.

CONSTITUTION: The surface of the photosensitive drum 1 is detected by the photosensor 4 prior to the copying process and its output is read. A microcomputer 5 compares its output with a previously stored detection output to select one of transistors Q1WQn-1 according to the drop in output level, and increases the load on a photodetecting element to perform output increase control over the element 3. In the copying process, another control system outputs a timing signal when a density pattern faces the sensor 4 and the sensor 4 detects the density of the surface of the drum 1. Its output is read in the computer 5 through an A/D converter 7 and compared with a reference density output value. The density control is carried out on the basis of the comparison result.