An image forming apparatus having a developing device wherein a developing material of a toner and a magnetic carrier is circularly transported from a toner supply portion to a developing region formed between the developing device and a photosensitive member while mixing the developing material. The developing device is provided with a sensor for detecting a toner density of the circulating developing material at a specified detection position in the developing device, and the toner is supplied from the supply portion to the developing material at the specified detection position and to the developing material at a minimum of one position other than the detection position when the detected toner density of the developing material is less than a predetermined standard density.
