PURPOSE: To make the output of a sensor in a standard state coincident with the control level for a prescribed image density, by making the gains of the output for the developing bias of a developing device, the density of a sensor pattern, and the density of the pattern image of the sensor variable and changing 1 thereof or above.

CONSTITUTION: The density of a sensor pattern 2 is set at 0.8:OD in a standard state by considering that the sensor pattern is provided on the bottom surface of contact glass. The developing bias is set at 200V by a switch SW1 and the impressed voltage to the light emitting element of a P sensor element 13 is set at 1.0V by a switch SW 2. If the toner density of the developer in a developing device 12 is normal, the output from the element 13 coincides with the prescribed value set with respect to the density of the standard image if the variations in characteristic and assmebling of each unit such as electrostatic charging, exposing and developing are perfectly adjusted in the stage of production. When the output value from the element 13 differs from the set value, a toner replenishing signal is emitted and the toner density, and electrostatic charging conditions are corrected according to the difference.