The object of this invention is to provide an image forming apparatus capable of preventing decrease in image density at a rear end part of an image. A change in waveform in sensor outputs which is produced when a toner patch image is read by an optical sensor used for the process control is detected. The difference (Vg−Vd) between grid voltage Vg and development bias voltage Vd is changed only between a target value of 300 V corresponding to low-output side threshold value Va and a target value of 100 V corresponding to high-output side threshold value Vb according to the sensor output deflection &Dgr;V so that the difference (Vg−Vd) decreases as the image loss level increases. The difference between the grid voltage Vg and the development bias voltage Vd is set so as to prevent the image loss in the rear end part of an image.