According to the present invention, an image forming apparatus comprises an image forming unit for producing an image on paper, a detector for detecting a plurality of parameters indicating a state of the image forming unit, a corrector for correcting the state of the image forming unit, a rule memory for storing rules for determining the relation between the parameters and the timing of operation of the corrector, and a control for controlling the corrector according to the parameters the detector detects and the rules stored in the memory. Therefore, even a simple configuration permits high-precision control. This reduces a time loss due to the correcting operation, minimizes consumption of development agent, and thus ensures constant gradation all the time.